Yes, you're right, the vibration is still there going down the road empty when I get up to speed. I like your idea that the gear shift lever would vibrate if the problem was the bad mounts! Since I don't feel any difference there, it seems that bad mounts are not the problem. It isn't a tire either or the vibration would not stop by putting in the clutch. So it seems that it is likely the U-joint. Or I guess it could also be a problem in the differential.
