Vibration can be a real pain to track down. I have replaced (because they needed it) tires, ball joints, tie rod ends, center link, control arm bushings, springs, and swapped the drum master cylinder to a disc master cylinder, balanced the wheels/tires, had an alignment done and rechecked, checked for bent rims and runout on tires and rims, and yet I have a 45-50 mph mild vibration. It does not affect ride or handling but it is enough to be perceptible. As it is felt through the seat and floor rather than through the steering wheel, I believe it is probably a drivetrain/driveshaft vibration. The u-joints are the factory nylon-sealed parts and undoubtedly are due for replacement. We put a couple hundred miles on today, and a hundred Dream Cruise weekend, so it's not an intermittent issue.