Lumpty wrote:
My guess is something driveshaft related, either the slip yoke at the transmission connection, or on many long wheelbase trucks there is a constant-velocity joint in the center, or a carrier bearing there, that may have some play.
I have experienced that on my 2500 GMC pick-up when towing my trailer. I removed the driveshaft and lubed the splines and it's been good for many miles. I had a clunk sound when moving from a stop.
The OP hasn't said much regarding his experience with all things mechanical so hopefully something said here helps.
