The problem with the 8 1/4 rear is the fact that the axle shaft is the inner bearing race and is subject to wear like Rusty said, but the 8 3/4 which uses a unitized bearing assembly be it the tapered roller or aftermarket ball bearing style the axle isn't in direct contact with the roller parts of the bearing. There is a "problem solver" bearing available for early 2000's jeeps that will fit 70's era mopars with worn axle shafts, it places the bearing on a normally unused part of the axle shaft if you axles are worn.
