by Trail X » Thu Jul 12, 2012 8:08 am
More information needed. Go through ALL modes of transfer case operation (try LO after you have determined the speed which it happens in HI, see if the speed is the same or different), try to determine what 4WD settings you see it in, and which you don't. What is the frequency of the vibration? Is it the same as the wheel's rotation? Faster?
Which driveshaft was Firestone talking about? If the front, inspect it for yourself. That may make sense if you only feel the vibration in 4WD modes.