My Craftsman YTS4000 (917.289281) Lawn Tractor drive belt vibrates really bad when I take the foot off the clutch/brake. I have replaced all of the 3 drive pulleys and drive belt but the problem did not go away. The vibration is pretty bad, to a point where the tractor can not be used. Can anyone suggest on what this could be. I am only talking about the drive belt vibration, the deck is off the tractor. Thank you