When I accellerate pass 55 mph the car shakes real bad until I release gas pedal

i have the same problem....i have had it balanced,fitted new tyres,replaced two motorsupports, replaced the front disc brakes but nothing helps...it shakes real bad from about 50 mph till 75mph and only when you accelerate. below or over that speed everything is fine......for my money it has to be the automatic gearbox or both the driveshafts....i dont want to spend a lot of money on driveshafts only to find it was the gearbox after all...anybody here who knows more.....please advise!