1. home
  2. how to
  3. transmission drivetrain
  4. How-To: Build an Ultimate Jeep XJ 88 Rear Axle

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

Ultimate XJ 8.8 Rear Axle

Bruce W. SmithPhotographer, Writer

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.

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.

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.

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.

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.