So when the accelerator is pressed and speed is increasing there's vibration coming from the rear of the car. If I let off the accelerator (and coast) vibration goes away. Any idea what this can be?

1991 GT, stock (cept for CAI, Flowmasters, 17" Cobra Repl)

New HB (less than a year)
New Clutch (less than a year)
New Engine Mounts (less than a year)
New Tranny Mounts (less than a year)
New Clutch Cable (less than a year)
New Tires (less than 6 months)

 

check the rear u-joint also make sure driveshaft bolts are tight and check pinion bearing for any noise on accel or cruise