We went to Reider Racing for the Precision Gear Master Install sets, and the Precision gearsets. Make sure to order inner axle seals for the front (they aren't included) because this is a great time to replace them. ORU was able to get us a pair because ours weren't in the best of shape.
While we were lifting and painting our '88 MJ, we were still undecided about what it was going to be: Grocery getter, back-halved rock buggy, prerunner, or what. Well, we've decided that it'd make more sense for us to use it as a prerunner, daily driver, and for light pickup truck duty rather than anything else.
With that in mind, after lifting it and running the 285/70R17 BFG MTs for a while with 3.07 gears, we were going nuts. The BFGs had broken in to be just over 32 inches tall, and at 2,000 rpm, we were doing 80 mph. That worked out for a nice cruising speed and meant that we weren't getting run over, but it was still lugging the engine and we had to downshift for some hills.
For some reason, downshifting for hills when talking gearing with most magazine editors is laughable. It's as though downshifting is evil once you are on the highway. Perhaps they've never downshifted to pass people or just to speed up quickly. Gearing around here eliminates Fourth as a passing gear, the rpm go way up in Fifth, and that's that (no downshifting needed).
Some of us aren't afraid of downshifting, and don't want to run an engine at 3,000 to 3,500 rpms just to keep up with traffic on the highway. Rather than go with the 4.88s, everyone was recommending we go with 4.56s in the MJ. We are planning on eventually going up to 35- or 36-inch tires and the 4.56s will put us at about 2,600 rpm at 75 mph, which is about all we'd want to cruise at anyway. With the BFGs we have now, we are at 65 mph and 2,500 rpm, which is about as high as we feel comfortable running it for long distances.
While we are able to set up gears, it's neither quick nor particularly enjoyable. Setting up gears is one of those practice things--the more you do it, the better you get. The last set of gears we worked on was more than a year ago and since the MJ has become the daily driver, we didn't want it down for however many days it was going to take us to do it.
We took the truck and our pile of gear parts to Off Road Unlimited in Burbank, California, for the install. Not only are they quicker than we ever were, but we were confident of getting the gears set up right.
Taking the carrier out is often a bear of a job, but you don't really need a case spreader to do it. Albert at ORU did our install and showed us how to get that pesky carrier out of the housing without a case spreader. | 
We went with a Detroit Locker in the rear. We didn't think that blasting through dunes and possibly muddy forest roads would be the best place to go playing with levers and buttons to lock and unlock the rear. So the Detroit does the unlocking and locking jig for us. | 
One of those things that comes from experience is a feel for the bearing preload. By tapping the carrier in with a dead blow hammer, Albert was able to get the preload on the bearings that were needed. |
The swapped-in, high-pinion Dana 30 front axle got a Detroit Truetrac limited slip differential. We didn't want to deal with levers up front either. However, a full locker in the front can sometimes reduce the turning radius. At speed that means that the truck would be likely to push through a corner. So we sacrificed some traction in favor of better cornering. | 
North County Jeep supplied the new speedometer gear. The parts staff was able to figure out that the two-wheel drive BA/10 gear was the same as the gear that goes into the NP231. After regearing, our speedometer claimed 75 mph at 2,500 rpm. Once we swapped the 32-tooth gear for the 37-tooth gear, our speedometer was right for the first time since we owned the truck. | |