I think it's just the carbide balls that flatten, as long as the diff feels smooth but not loose you will be fine
Don't forget the threadlock on the alloy motor mounts as well, there is also a thread in the associated section about the steering assembly locking up if not put together exactly right, didn't have a problem myself as got mine second hand but worth a look before you get to it
Any more probs just pm me mate & enjoy building it