'99 Suburban 4wd 5.7L with 200K miles... Due to a failed thrust bearing in my transmission (according to the dealer), I had my transmission rebuilt. All seemed great until I took it on the highway where it began vibrating in the seat at around 60mph, then peaked at around 65mph and diminished as I got faster. I took it back and the tech drove with me and concluded it was a U-joint. I told him that the two other times that my rear U-joints failed the vibration was at 50 mph, not 65. In any event, they replaced the rear U-joints and alas the vibration did not go away, so I took it back once again. They had it for several days and concluded it was the rear differential that needed to be rebuilt for $1100. Well, I just spent $2000 on the transmission and U-joints, and I don't have a good feeling about their diagnosis. Wouldn't I heard whining or gnashing if the rear end was bad? I hear no noises. The car runs quietly. Does the front driveshaft spin with in 2WD mode (not auto-4)? Could it be that shaft? It vibrates in all drive modes (2wd, auto-4x4) and even when in neutral at 65mph. Since I didn't have a vibration before the transmission rebuild, what could they have touched that could cause this problem? I need help... Thanks!