Vibration in drivetrain

sure it's the drivetrain and not the front end. Jack it up and set the rear axles on jack stands where tires are off the ground.(Car pointed in a direction where it won't hit anything if it falls off). Put it in drive and slowly rev it up to 70-80, still vibrate? And please don't have anyone stick any body parts under car while it's running to look around - slide a mirror or something under there.