We need lockers and gears, and we not only need them, but we also expect them in any decently built four-wheeler. Certainly our Project Xterra, fit with extra horsepower and appropriate wheels and tires, was turning into a reliable, quality trail rig, but as we slipped over rocks and struggled up gnarly inclines, what we intrinsically knew became blindingly clear: It needed lockers and crawler gears.
Also, while we were whipping lockers into the differentials, this would be the perfect opportunity to swap out the X's stock 4.36 gears for a set of 4.88 gears from Nissan Motorsports. The ratio is a bit low, really, for our 31-inch Baja Claws, and makes the Xterra a bit buzzy on the highway. But it improved the X's crawl ratio to 35.3 from the stock 31.5. Add to that a significant reduction to the Xterra's low-range ratio and what we now have is a crawl ratio of 68.5:1.
To enhance the X's streetability, the lockers had to be selectable. We called ARB, knowing the company made a rear locker for the Nissan. Serendipitously, it turned out the company was developing a front locker for the Xterra. We got our hands on a prototype and headed over to the pros at Tri-County Gear, in nearby Pomona, California, to install the new lockers and gears.
Setting up new gears is truly a science in and of itself, requiring special tools and plenty of know-how. Unless you're schooled in the task, don't try setting up a diff on your own. As one of the Tri-County pros told us, "Proper setup can make the difference between your differentials lasting 3,000 miles or 100,000 miles."
Once we got the gears and lockers set up, we turned our attention to the Xterra's transfer case. Nissan specialist CalMini recently introduced a 3.92:1 low-range gear reduction for Nissan transfer cases.
We got a set, had the techs at Four X Doctor drop them in and hit the rocks. Here's a chronicle of our time at the shops. Special thanks to everyone at Tri-County Gear in Pomona, Off Road Unlimited in Burbank (ARB air compressor) and the Four X Doctor, also in Burbank (T-case gears).
Front Diff
The IFS of the Xterra features two halfshafts, making diff removal easier than you might expect. Drain the pumpkin, remove the front wheels and unbolt and remove the skidplate. Remove the front crossmember. To access the differential mounting bracket, we had to pull our steering stabilizer. Unbolt the differential from the bracket and disconnect the halfshafts. Don't forget to unplug the vent hose, a commonly overlooked step. Finally, unbolt the driveshaft and pull the differential. | 
Pull the axleshaft and pry out the carrier and ring-and-pinion. We retained our carrier's bearings, since our Xterra had just 15,000 miles on it. The lip of the diff housing where the pinion seal fits must line up exactly when you're done. Here, our mechanic measures the preload on the gears at 5 in-lb. Since we're keeping our original bearings, this was the rating we'd work toward with the new ring-and-pinion. With new bearings, we'd set preload somewhat higher, probably at about 10 in-lb. | 
Remove the pinion yoke with a 1 1/16-inch socket. Save the shims on the pinion. These help set the preload rating. When installing the locker we used the OE bearings. If you use the same gears, make sure the height where the new ring gears sit matches the old carrier height. Since we were unable to pull the pinion race (a sleeve that fits between the pinion and the housing), the guys at Tri-County used a bearing spreader. |
Our mechanic press-fit the bearing onto the pinion. After lubing the race inside the carrier, he gently taps on the pinion, being careful not to damage the pinion seal. Once in, tighten the pinion yoke so that the pinion does not "walk." You may tap it with a rubber mallet to get it seated properly. | 
Take the ring off the factory carrier. Using the OE bolts, bolt the new ring to the ARB carrier and bearings. Then, press the bearing onto the carrier housing. Grease the ring gear and position it in the carrier. Spin it a few times by turning the pinion yoke. The grease should spread evenly on the teeth of the ring gear. The backlash, or amount of play on the housing, should measure 0.008 inch. This is achieved through trial and error-a trying process that illustrates exactly why such installs should be left to the pros. | 
Finally, install the air line, making sure it doesn't touch any parts, especially the ring gear and the carrier. Work carefully not to crimp the line. Attach the two provided ferrules. The one with the flat end fits on the outside of the tube. Test the pressure in the line. Our prototype needed an elbow to fit outside the carrier that would connect it to the air lines, so the guys at Tri-County fabbed one up. (ARB will include all necessary parts when this locker goes into production.) Once preload and backlash are adjusted to their proper settings, return the axletube to the diff and reattach all the frontend parts. |
Rear Diff
Start by removing the wheels and disconnecting the driveshaft and brake lines. Unbolt the diff housing and pull the rear carrier. Disconnect the main pinion seal. Pull the yoke nut and separate the pinion from the housing. Then remove the OE ring gear and keep the bolts for the ARB housing. | 
Install the ARB carrier and tighten the bearing cap. Again, using trial and error, adjust the backlash of the carrier so that it can move only 0.008 inch. Install the air line and bend it to the exiting hole. Return the carrier to the housing and reinstall. | |