Generally speaking, the ’87-’95 YJ Wrangler is the red-headed stepchild of the Jeep world. This is the open-top Jeep everyone loves to hate. Maybe it’s because it replaced the CJ, maybe it’s because it had square headlights, maybe it’s because YJs from the factory look low and incapable off-road because Jeep was first realizing how bad anti-rollover press could be. Luckily for YJ owners, once you clear a few hurdles the YJ is a great rig to own and wheel. We generally tell people to shoot for a ’91-or-later model with a 4.0L with a three-speed auto transmission or the reliable AX15 five-speed manual. Before 1991, you may be best off with a four-cylinder just because they are fuel-injected, and the 258ci I-6 powered YJs before ’91 were plagued with a horrible computer-controlled Carter carburetor. Other YJs to avoid include ’87s with a factory NP207 and possibly the Peugeot BA10/5 manual transmission or ’88-’89 with the 258 I-6 and the Peugeot BA10/5. Otherwise, all YJs share a strong frame (when not rusted into oblivion), most have galvanized sheetmetal that is more resistant to rust than the ’87 YJs and earlier CJs, a decent leaf spring suspension, and decent front axle. The only rear axle found in the YJ is unfortunately the Dana 35, which we pretty much never advise wasting cash on.
Building a YJ for mud is a fairly standard recipe. Add some suspension lift, big tires, and lockers to your 4.0L YJ and you are good to go. If you are building for 33s or 35s, you might be able to get away with keeping the Dana 30 front axle, but the rear Dana 35 will need to be upgraded for tires over 32s. If you have a four-banger YJ and live in an area where emissions are not an issue, upgrading to a GM or Ford small-block V-8 is a popular option. You’ll also probably want to swap out that light-duty AX5 manual for an auto or manual tranny that can handle the V-8. Rebuild that NP231 with a wide-chain kit, six-pinion planetary, and a slip-yoke eliminator from a source like JB Conversions, and you’ll have a pretty solid drivetrain.
As far as axles you could gather a ’80-’84 driver-drop Wagoneer front Dana 44 and a matching six-lug Isuzu Rodeo/Honda Passport Dana 44 rear with disc brakes for your YJ. Those should be easy to swap in and good in mud up to about a 37-inch tire. If you wanted to go full-width or to 1-tons, look for driver-drop Ford Dana 44 (with welded on, not cast in radius arm mounts) or a front and rear Dana 60 front, rear Ford 9-inch, or for maximum beef, a GM 14-bolt.
Keep it low and light and you can have lots of fun in a YJ in the rocks. Since the trend is a low lift with big tires we’d recommend a high-clearance hood like the ones from Chris Durham Motorsports or 4 Wheelers Supply. Then run a spring-over suspension in the front. Out back, get to hacking and trimming for bigger tires so you can run a 2.5- or 4-inch rear spring-under suspension to help fight axlewrap. Add in a good rollcage and some stout rocker guards and your YJ will be safe and protected from the rocks. If you want to get fancy, you can run stock FSJ front springs backwards in a spring-over configuration and some 4-inch FSJ front springs out back in spring-under for a little bit of stretch for those waterfall climbs. You might have to drop the factory gas tank, but you could go to a fuel cell in the bed or a new tank designed for a wheelbase stretch.
If leaf springs are just too old school, you can swap to one of many three- or four-link suspension kits and run coilovers or air shocks in the front or rear. Any of the factory engines can turn big tires with the proper gearing and any transmission with the exception of the Peugeot BA10/5 will be fine. Go to a 4:1 kit for your NP231, add an auxiliary low-range box, or swap to an Atlas II or Atlas four-speed for more gearing than you’ll ever use.
As for axles, the same basic information holds true for rocks as for mud, but lockers are a must. You might want to drop a tire size, as rocks and big tires are hard on drivetrain. If running 35s or 37s, you’ll need Dana 44s at least, want bigger tires, and we’d say going to 1-ton axles with 35-splines or more are a must.
You are definitely gonna want a 4.0L or a 258ci with a good carb or fuel injection on it, if not a V-8 swap. Sure, you can still play in your four-cylinder powered YJ, it will keep trying, but it’s gonna hate you for it.
In the sand you’ll quickly find the limits of the YJs fairly plush leaf-sprung suspension. You can try using longer springs with a shackle reversal up front or swap to a link style suspension and coilovers, but the short wheelbase will eventually limit the upper end of high-speed prerunner play in a YJ. If dunes are more your flavor, a YJ would be a great base to build-up a sand drag Jeep with its durable boxed frame. Just drop in massive cubes, a built auto tranny, front shackle reversal, a strong rear axle, a linked, coilover suspension, stout roll-cage, mix and pour.