I presume you mean the ring and pinion gears. (the little one in the bulkhead and the big one around the diff case.) Shimming is by feel... it should roll smooth without binding, but otherwise be a good fit with minimal lash. Too loose = lest contact patch and more wear. Too tight = binding and more wear. Also you should grease the gears every time you do your diffs... we use lithium grease (white stuff in a tube.) Just enough to get a good coating all the way around the ring gear.