All great points, and the probable cause is likely in one or more of the options offered, but I would add one more thing to consider: faulty bearings. A lot of aftermarket stuff is made to look like O.E.M. nowadays, but falls well short of original spec as far as materials are concerned. If your thrust bearings were made of inferior material, they could have failed way early simply due to not being able to hold up to "spirited" use - or even little old lady driving. If all of the clearances check out, I think inferior bearings might actually be the simplest solution.