Seaball, I may be wrong, but I think it's the thrust washers and balls that usually get gritty. If you think about it, they have all the pressure of the diff screw and spring on them, they are much smaller and have less balls which means they carry an increased load per ball compared to the diff balls and rings. Usually you can clean the diff, balls and rings, replace the thrust ball/washer assembly and your diff will be good as new.