De-bugging a "seat of the butt" shimmy at 55-65 MPH. truck is 2WD, 175K miles, 98 12 valve Cummins with the auto transmission. U joints are all good, appear fairly new, pinion bearing seems good, with the normal slight weeping that means I will have a seal job soon, but I do notice a slight amount of side play in the trans tailshaft. I'm thinking this may be a the problem, unless I have a bad belt in one of the sneakers it's the only thing I can see. Is a very small bit of side play normal, or should this be dead solid? And, I do have a Federal seal/bushing kit here from out old '94 that was sold before we foxed it, would it be the same? Any one know the Fed part numbers? Working on retiree's funds so just dropping it at the shop is out of the question. How involved is the seal/bushing replacement? Thanks for the help.