98' Blazer 4x4 automatic. Im having a drivetrain problem. It vibratesloudlyfrom the rear when accellerating hard from a stop. And drives fairly quite around town driving easy. The vibration comes back at high speed ( 65-80) and sounds like I have somebig *** stereo subwoofers. It is like ( Whooooo,Whoooooo,Whoooooo,Whooooo). . I have checked the U-joint's and wheel bearings and both seem OK.Any body ever have any problem similar or any tips on what to check.

 

Mine does the exact same.
If I put it in 4WD, the vibration is unbearable...Then as I shift out of 4WD, over the course of a week the vibration subsides to the woob, woob woob noise

 

Sounds a lot like an out of balance driveshaft, but some people on this other site's thread looked at brakes and u-joints as well:

http://www.automotiveforums.com/t521019.html