I have an 04 Sierra 2500hd and have been chasing this vibration. It vibrates at 70 mph and up. It's weird if your on the gas it vibrates like crazy if you let off the gas it stops then when it starts to deaccelerate it comes back. You can also notice the vibration at lower speeds but it's not bad if you weren't looking for it you won't notice it but at 70 it's real bad. I've had the drive shaft balanced new u joints new slip yoke and the transfer case rebuilt with no change everything feels tight it also does it in lock up or out of lock up I'm not thinking it's the tranny. The tranny has been serviced shifts fine. Any ideas