2001bowtie

Hello CF,
My name is Zach and I have a 2001 Chevy Silverado 4wd. It has a 6.0 in it and it also has the NV4500 5 speed manual transmission in it. A couple weeks ago i noticed a vibration coming from the undercarriage. The vibration would happen whenever i would hit about 70 mph accelerating and mainly whenever i would let off the*gas*and coast at that speed. I replaced the rear drive shaft U-Joint that goes into the transfer case and that took care of the vibration whenever i would let off the gas and coast. But now it still vibrates whenever i accelerate around 70mph or higher. The vibration will vibrate under the drivers seat when it happens but once i stop accelerating it stops vibrating. I can not only feel it but the passenger riding with me can also hear the vibration going on and so can I. I do have 33s on the vehicle but it is not a*tire*or lift issue I do know that. Any ideas??

Thanks in advanced for your input and help!