Mine was doing it when I was taking off from a stop making a left or righ hand turn...There was a pronounced shuddering and noice coming from the rear and my tires were starting to show pre-mature wear...They (Ford) finally replaced my pumpkin as well as one of the half-shafts in the rear...The unfortunate piece is that the new Pumpkin had the service pack installed in it... Mine was all covered under warrenty, so I can't help you on the costs...Ford Customer Service, placed a complaint on my behalf for the poor engineering design and Ford Engineering was going to look into it....