For the sake of time I will skip all of the disassembly leading up to taking the gearbox out and start from the point where your gearbox is out of the chassis.

    With the gearbox sitting “assembled” on the table (Drive cup still installed), grab hold of the drive cup and lightly push it into the gearbox, then pull it back out. This slop you feel is "some" of the problem with the gears skipping. This is allowing the pinion to move away from the ring gear while driving. Now remove the grub screw and take the drive cup off of the pinion shaft. Separate the two halves of the gearbox. Slide the pinion/shaft out of the case. Wipe all of the factory grease from both the ring and the pinion gears. Make sure to get as much of the grease as you can out from between the gear teeth. Now look at your pinion gear/shaft. You will see approximately two shim washers that are already on the pinion shaft from the factory. They will be pushed up against the back side of the pinion gear. If they aren't there, they may have stuck to the bearing and not came out with the pinion shaft. The next step is to place additional shims on the pinion shaft. Some people have made their own out of coke can material (I would not recommend this as they are aluminum), and others are ordering replacement shims (The best way to go if you can locate some). I made my own out of a piece of .005" stainless shim stock. The amount of shimming needed will be different from truck to truck. I would start with one additional shim at a time until you get the proper gear mesh between the ring and pinion gears. How can you tell if you have proper mesh? Here is the method that I used...Put your shims onto the pinion shaft (factory supplied shims plus one additional shim) and slide the pinion shaft back into the support bearings. Re-install the drive cup. Before snugging the grub screws, lightly press the drive cup onto the pinion shaft holding one finger over the end of the pinion gear and the other over the end of the drive cup. It is very important to do this to eliminate any slop between the pinion gear and the drive cup. Snug the screw up. Note: Check to make sure that the grub screw is aligned with the flat spot on the pinion shaft before snugging. Put the two halves of the gearbox back together and spin the drive cup. Make sure that it spins freely without any roughness. Repeat this process of adding one shim at a time as per the process outlined above, until you can feel that the ring and pinion gears are beginning to make contact. When the ring and pinion gears begin to make contact they will become very rough when you spin the drive cup. When this happens you are shimmed too tight. At this point, remove one of your shims and pack some grease between each of the remaining shims to help reduce wear. Reassemble the box. Make sure to apply loc-tite to the drive cup grub screw before final re-assembly. The gears should again spin freely without grinding against one another. Make sure that they spin freely the entire way around the ring gear. Apply a good quality black moly gear grease to both the ring and the pinion gears before re-installing the gearbox onto the chassis. That's it. Hope I didn't forget anything. Again, that is the method that I used and it worked fine. I had to add 4 additional shims (.005" each) to my front gearbox and 3 to the rear.

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