I really doubt that you'd buffed back that much on the diff rings, but if the initial thought by most people of the thrust unit failing. Would either cause the diff to spin apart and have zero grab, or be completely locked. But your problem sounds aas though its consistently slipping. If so, my only logical explanation at this time is that the halves are not clamping effectively. Most probably by the two support bearings. Those bearings should never be subjected to laterall loading, hence the fact that they are ball bearings not thrust type. They are there to give the halves and the pulley something to rotate on. Check them out. It might be a bit hard to remove the bearing in the diff halve though.