Shelbycharger's description is a pretty good explaination of what you want. I have not rebuilt a diff. on a skidder, but the principles involved are the same on any diff. shimming the pinion for correct rotational "drag torque" is important, if it is too little, you will have premature bearing failure, and if it is too tight, you will result in heat buildup at the bearings and they will fail as well. Make sure when you are checking the rotational drag on the pinion that you have some oil on the bearings, there is a big difference on rotational drag between wet and dry bearings. The correct contact pattern needs to be checked with layout dye, (dykem, or prussian blue_). You want the contact area to be center top to bottom on both the pinion and ring gear teeth, and to be slightly toward the center of rotation on both gears, or toward the "toe" of the gear tooth. Shims need to be adjusted as required for proper gear tooth contact. If you are looking at the gears as they were removed, unless you had to change parts due to being badly worn, you should be able to see where the contact pattern was before, and re establish it in the same area. finally be sure an check for proper backlash, (free movement between the 2 gears with no contact), should be a few thousaths of an inch. I am not sure of their precision spec, but guessing on a range of .004"-008" Maybe a forumite will be able to link you up with the exact procedure from the rockwell manual. Good luck, http://www.precisiongear.com/pgtechpatterns.htm
this link shows some pictures of what I am trying to describe for gear tooth pattern.