Generally you add shims to the ring gear side to tighten up the mesh between the ring and pinion. I thighen mine up until they feel slightly notchy. Then just let them wear in.

The part I was talking about earlier was inside the aluminum diff cup / housing. There should be thin flat washers behind the two large bevil side gears, and behind the 4 small spider / bevel gears. There should also be an o ring behind the. Two side bevil gears and flat thin washers. They seal the drive cups and keep grease from leaking out around them. If they are missing that could also cause excess play.