On my '03 ECSB with 170k on it, I got a vibration that comes from it whenever I'm on the highway or interstate. It always starts in at about 75 or 76 mph and does it all the way up to the low 80s and if you go even faster up to 85 or more, it goes away. And whenever the vibration comes on, it does it when the pickup is under a load, such as accelerating up to speed or pulling a grade/hill. It goes away when you're coasting and comes/goes when you're on a level drive. Does it sound like a U joint or is my rear driveshaft out of balance. I know at some point both the rear U joints have been replaced. I haven't taken the time to pull the rear differential cover off to inspect the diff and change the fluid. It couldn't be a front hub, and somebody correct me if I'm wrong, because it would be making more of a grinding noise right? Any help is appreciated.
