Hold the shaft (no jokes here guys) and rotate the diff back and forth try to feel the "lash" and tighten up by shimming. The noise also increases quite a bit when even a hair tight.
The diff is most of it and you might find to remove the side to side play you need to put shims on both sides of the diff I think my front box has .1 on the battery side with .4 on the other and .2 on the pinion.