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How-To: Build an Ultimate Jeep XJ 88 Rear Axle

Posted in How To: Transmission Drivetrain on April 5, 2019
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The popularity of Jeep XJ Cherokees (1984-2001) is continuing to grow as off-road enthusiasts find more and more reasons to fall in love with one of Jeep’s most popular SUVs of all time. The Unitbody design, roomy interior, driving comfort, off-pavement prowess, and reliability of the 4.0L inline-six attracted more than 2.1 million buyers over its 17-year run.

Those traits, along with the vast array of aftermarket products available to take the XJ to high levels of off-road capability, have owners refreshing, rebuilding, and in some cases reconfiguring beloved Cherokees that are getting a little long in the tooth. Mike and Sydney Davis are among that group. They bought their Cherokee Sport off the dealer lot in 1998. Since that time it’s been Sydney’s daily driver, pampered and garaged, and driven on street and trail all over the Western United States. Now, with more than 165,000 miles under its wheels, “Her Jeep,” as Mike says, is getting a new lease on life instead of heading for early retirement.

To make the rearend swap, the shocks were removed, leaf springs were unbolted from the XJ’s Unitbody frame, brake lines were disconnected and capped off, and then the Cherokee was raised and the rearend assembly rolled out of the way so the Explorer 8.8-inch could be put in its place.

Bigger Rear Housing

Among the list of changes is upgrading the factory Chrysler 8.25 axle assembly to one better suited to handle the load placed on it by 35s and the forthcoming more serious wheeling. The 29-spline factory axles in the ’98-up XJs are secured by C-clips. These 29-spline shafts can hold up to 35s unless one is really abusive, but the C-clips are prone to breaking under severe loads, leaving one three-wheeling or worse.

Instead of rebuilding the Chrysler 8.25, the techs at Dunks Performance turned to the popular Ford 8.8-inch assembly that’s found under a wide range of Blue Oval pickups and SUVs from the same era. The ideal axle for the swap is one sourced from a ’95-’02 Explorer equipped with disc brakes.

A ’97-’02 Ford Explorer rearend is the ideal candidate to replace the Chrysler 8.25-inch found under XJs. It’s the right width (59.5 inches), has the largest axletubes (3.25 inches) of the 8.8s, runs 28- or 31-spline axleshafts, has a 1/2-inch-bigger ring gear than the Jeep’s, and it comes with disc brakes.

Those assemblies have a 59.5-inch flange-to-flange width (1 inch narrower than the XJ’s Chrysler 8.25) and sport the largest (3.25-inch) axletube Ford offered in the 8.8s. They are abundantly easy to find, and we’ve seen them for less than $300 in junkyards. It is important to note that Explorer 8.8s came with both 28- and 31-spline versions; the latter is harder to find.

Fixing Weak Links

The caveat with swapping in an 8.8 underneath the XJ Cherokee is that the Ford shock and spring mounts need to be removed and new ones welded on to match the Jeep’s configuration—and if the internals are left stock, the Ford axles still utilize the weak C-clip design.

Dunks addressed all the above issues by taking the rear axle from a junkyard ’98 Explorer, stripping it bare, then installing a Yukon Gear & Axle Ultimate 88 axle kit. The Ultimate 88 kit eliminates the C-clips, replaces the stock 28-spline axle with much stouter 31-spline 4340 chromoly shafts, increases the overall axle width by 1 inch, and allows using wheels with a 5x5.5-inch bolt pattern.

To make the junkyard Ford 8.8 bulletproof for running 35-inch tires off-road, it required a few upgrades: Yukon Gear & Axle’s Ultimate 88 chromoly axles, an ARB Air Locker, Nitro Gear & Axle’s ring-and-pinion, and new backing plates from Currie Enterprises.

At the same time, a Yukon disc brake kit, with Currie Enterprises’ backing plate assemblies (they have the parking brake), was swapped in to replace the Ford disc brake setup. Then new shock brackets and leaf-spring perches were welded back on to align with the XJ’s 6-inch BDS lift and BFG 35s.

The only other potential weakness of the Ford 8.8—in strength and traction—is the differential. That concern was alleviated with a Nitro Gear & Axle 4.88 gear set mated to an ARB Air Locker. The low gears offset the taller tires, so the 4.0L and factory transmission still retain the Cherokee’s original get-up-and-go. The Air Locker brings bulletproof reliability and an instant boost in traction when needed.

Replacing the Chrysler 8.25-inch rear axle assembly with a Ford Explorer 8.8-inch modified this way is a fine setup for someone wanting a well-prepped, trail-bound XJ Cherokee that’s being taken to the next level.

The Ford 8.8-inch differential is considerably stronger than the XJ’s Chrysler 8.25-inch. But it still has two weaknesses: The C-clip design, which the Yukon Ultimate 88 kit will eliminate, and a differential carrier prone to cracking or distorting under heavy off-road loading, which an ARB Air Locker can cure.

Beefier Dana 30

Of course, setting up an XJ for more difficult off-road encounters requires beefing up the front Dana 30 as well. Dunks Performance went with 30-spline chromoly axleshafts to replace the factory 27-spline sticks, put in manual-locking hubs, and installed an ARB Air Locker. Yukon Gear & Axle’s Spin Free Locking Hub kit was designed specifically for such a rebuild.

Yukon’s Spin Free kit replaces the vulnerable and expensive Jeep unit bearings with tapered bearings and races, along with everything needed to upgrade to the stronger axleshafts and manual hubs to make serviceability easier and less expensive, all the while improving fuel economy. The kit includes new wheel hubs (5x5.5-inch bolt pattern), spindles, high-strength Yukon 4340 chromoly outer axles, premium locking hubs, Timken bearings, high-quality seals, and all the hardware for installation. It also carries a limited lifetime warranty against manufacturer defects for as long as you own the vehicle.

While Dunks techs had the Dana 30 disassembled, they replaced the ball joints and all of the suspension bushings, which showed the typical wear of 165,000 miles and 21 years on the road. They also installed a PSC gear box/pump to tighten up steering. The “Ultimate 30” will make the perfect companion for the Ford 8.8 out back.

After the Explorer rearend was completely disassembled, Dunks went to work cutting off all the brackets on the Ford housing and grinding the tubes smooth. New spring perch and shock brackets will be welded on after the refurbished 8.8-inch housing is moved back under the XJ.
Installing the Yukon Ultimate 88 kit, which eliminates the C-clips and replaces the stock 28-spline (1.29-inch-diameter) Ford axleshafts with 31-spline (1.31-inch-diameter) chromoly versions, requires cutting off the portion of the stock axletube that holds the bearing. The cut was made flush to the shoulder that extends from the axle flange.
It’s important that the tube ends are ground smooth and flush with the shoulder, and the inside of the axletube needs to be deburred and clean so the new Yukon bearing will align properly.
Special setup nuts are used to “press” the new bearing seat into the axletube on each end. Then, the installation is removed so the Currie backing plates (with e-brake) can be installed.
The Yukon kit holds the axles in place with a retainer plate, spacer, bearing seal, bearing, and bearing retainer hydraulically pressed onto the 31-spline chromoly shafts. This combo eliminates the need for the problematic C-clip.
Proper oil drainage where the axletubes enter the differential housing is critical with the Ultimate 88 installation. We had to cut a notch in the passenger-side axletube because it didn’t have a drainage hole to let oil flow out of the tube. Both sides need lower drainage slots.
The factory Ford carrier and gears were replaced with an ARB Air Locker and Nitro 4.88 ring-and-pinion. Everything was torqued to the aftermarket manufacturer’s specs. Beefier carrier and gears provide the strength needed to handle 35s in demanding off-road environments.
An ARB Air Locker requires drilling and tapping a hole in the top of the pumpkin so the airline fitting can be installed. After that was done, the entire housing was flushed to rid it of any metal shavings or debris before assembly began.
Proper gear setup is a big part of rebuilding any differential. This one’s final setup had 21 in-lb of pinion preload, 0.008 inches of ring gear backlash, and both drive- and coast-side gear contact patterns centered on the ring gear teeth.
Care has to be taken when routing the copper ARB airline to protect it from inadvertent damage where it enters the carrier—and so it doesn’t contact any part of the differential. Excess line is trimmed off flush at the base nut on the external fitting.
With the new axle retainers and Currie backing plates in place, it’s a simple process to bolt on the Yukon 31-spline chromoly axles that came with the Ultimate 88 kit.
The refurbished Ford Explorer axle assembly was rolled under the Cherokee and loosely bolted to the new BDS leaf springs. The aftermarket spring perches will be welded on after the wheels are on and the correct pinion angle is determined with the load of the XJ sitting on them.
Using an iMetalBox app on an iPhone is an easy way to view driveshaft and differential pinion angles. Once the 8.8-inch Ford housing was rotated to the proper 2-degrees-down angle for use with the slip-yoke-eliminator T-case (and BDS 6-inch long-travel lift), the spring perches were welded in position. Minor angle adjustments will be made with spring shims as the new spring packs settle in. Shock brackets will be welded on last.
The factory Chrysler 8.25-inch rear axle under this ’98 XJ Cherokee was getting tired, and the C-clip design with drum brakes just wasn’t going to cut it off-road running 35s.
Swapping it out for a completely reworked ’98 Ford Explorer 8.8-inch assembly with disc brakes, Yukon 31-spline chromoly axles, a C-clip eliminator kit, an ARB Air locker, and Nitro 4.88 gears was a far more robust alternative.
Dunks pulled the factory differential out of the XJ’s Dana 30 frontend and replaced it with an ARB Air Locker and Nitro 4.88 ring-and-pinion to match those in the rear differential.
It’s important to make sure there’s enough clearance between the track arm on top of the Dana 30 to clear the ARB air fitting. We would have been more comfortable with the fitting on our differential another half an inch farther away from the mount.
Yukon Gear & Axle’s Spin Free locking-hub conversion kit (PN YA WU-08) comes with everything needed, from bearings to chromoly 30-spline axlesshafts to rotors. We used the company’s 5x5.5-inch hubs.
Added trail strength comes from a bigger and stronger 30-spline axle Yukon chromoly shaft (left), which replaces the factory 27-spline that is standard in Dana 30s.
Along with the stronger 30-spline axles, the U-joints were upgraded to Yukon’s 760X 4340 chromoly Super Joints (PN 32081). These U-joints use a bushing design instead of troublesome needle bearings.
Yukon’s manual-locking hub conversion uses hat-style rotors where the wheel studs are pressed in to secure them to the wheel hub. This XJ was set up with a stouter 5x5.5-inch bolt pattern that’ll hold the 35s the owner wants to run.
We also took the time during the Dana 30 rebuild to replace the tired factory steering box with a PSC Motorsports Big Bore steering gear and XH high-volume pump kit. The 8200 series conversion steering gear is a direct replacement for the small piston Delphi steering gear that was used on ’97-’02 Jeep TJs and XJs. This larger piston diameter increases the torque output of the steering gear, making it much easier to turn bigger tires.

Sources

Currie Enterprises
Corona, CA 92880
714-528-6957
http://www.currieenterprises.com
Nitro Gear & Axle
Sacramento, CA
916-673-6464
www.nitro-gear.com
Yukon Gear and Axle
888-905-5044
www.yukongear.com
Dunks Performance
541-726-1006
http://dunksperformance.com/

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