Sounds like axle wrap. Do a search and read about it. Does it stopping and taking off.
Do you have the utilitrack system? The timing delay seems a little long, but the plastic covers on the bed floor tracks slide fore and aft. I always get a thunk after a stop. My thunk is the covers hitting the front of the bed.what said:I recently started noticing at stop lights that 5-10 seconds after coming to a complete stop it feels like someone came up and kicked the rear of the truck. The first time i felt it, i thought someone had rolled into me. Any thoughts on what is going on? I would take it in to the dealer but don't have the time right now.