Hi, I'm new to the forum. I have a 1998 Silverado 4x4 5.7l with 185,000 miles and I have a vibration at 75 mph that feels like its coming from the rear end. I was driving to CMU last weekend and when I would speed up from about 65 to 80 to pass a car it sounded like i went over a rumble strip. i cannot figure it out, i had this problem before and had to replace the driveshaft, both u-joints, both yokes and had the t-case sent out because that's what the shop thought the problem was, but they said it was fine. The tires have 6,000 miles on them and I just had them rotated and rebalanced and it didnt help. If anyone can I would really appreciate it.

 

Vibrations can be hard to tell where thay are coming from. I would check all the regular items - shocks, front end, etc. Worse case could be a bent axle. I just tackled a similar vibration and ruled out the tire balance because they were just done. It ended up that they were not done properly and that was my issue so unless you have already done so find a reputable shop that will make sure they "zero" out on the machine. I personally don't like the bubble balancers and make sure that mine are done on a high speed electronic machine.
Good Luck!