[Serum]

Check to see if there is no stuff in the bearing holes. use a toothpick to poke them out.

Are you using the 43/13 or the 43/10 diffs? if everything is tight in place, it's pretty hard to solve the problem. It either takes a deeper bearing hole behind the ringgear or behind the pinion.

The 43/13 diffs do fit in my diffs.

With no pinion in place; have you got any room on the diff inside the case? (4 screws tight in the diff, 3 screws tight in the bulks) try to see if you can move the diff left/right