It sounds to me like an alignment problem. The output shaft of the transmission must be parallel to the pinion shaft on the rear end. If they are pointing straight at each other there will be a problem. You need to take the drive line out. Then put an adjustable protractor on the rear end yoke where the U-joints would be attached. Write this reading down. Then do the same on the output shaft of the transmission. The rear pinion should be pointing down and the transmission should be pointing parallel to the ground. Then when the rear end "twists" up under acceleration these two shafts stay aligned with each other. If they do not then you get the vibration you are speaking of.