I see that this " THUNK " sound is pretty common sound in Ford Trucks.....I have a 2001 F-150 Crew Cab 4X4 with the same problem.....I have read post after post but really haven't got a clear meaning on any of them.....From take gradually accelerating my truck does fine.....but when I stomp on it and it down shifts thats when it goes into like neutral for a few secs then here comes the " THUNK "......I got under the truck and examined everything best to my knowledge.....noticed that there was a ring around the yolk where the driveshaft goes into the tranny that is shinney like it is going in and out of the tranny but I figured that was normal with the up and down of the rear end....its shinney about 2 incles from the tranny back towards the rear end....is the problem.....NEED SOME HELP