Don't know how you can have bad alignment of thrust balls??
Anyway, it is know to strip easily if you over tighten the bolt in, so be gentle when you set your diff, Asso had a problem with a serie of nuts but this was a while back.
Solution: use some threadlock on bolt, make sure your diff is properly built too, I have seen diff not built properly on numerous occasions, use black grease to hold your thrustballs in place and compress with pliers the spring when it is new.
If it is gone, change for a new nut, they are cheap.