vibration

hi folks
well I have had a vibration for years but to narrow it down roughly the vibration Is there under acceleration and stops when I let off the gas pedal. the vibration gets worse the faster I go and it is not in the steering wheel just the body of the car. my rear view mirror in the car is so blurred I can not see. the rear end was done by dr. dif so I feel the pinion angle is good. the u joints feel good. I did have the drive shaft balanced years ago but not sure how good of a job he did. the shaft length is 49 inches and I can push it in the trans I inch when I remove it from the rear yoke. this is in a 68 barracuda 360 crate motor 8 3/4 with a 4 speed.