I have what I call a shutter on my 96 automatic,reg cab dually. It is noticed most when I am towing a camper and am going 50mph and higher. It doesnt do it at normal cruising, only when Im going up a grade or giving it a little more throttle. The truck feels like I have a bad carrier bearing on the driveshaft, but I would think that if it was that it would be at any acceleration. If I let off the pedal it is ok, or if I give it alot more pedal it basiccaly goes away... Any thoughts,Thank you