It could still be ethier the u joint or pinion Pro,i've had alot of trucks myself that had the "clunk" when ya take off or let off the gas real fast and 9 times outa 10 its the U joint and the 10th time its the pinion lol

Back when i was into the truck/four wheelin thing alot of my "truck buddys" always told me the best way to check is to do what you did but to jack the tires off the ground when you do it so that everything "free" when ya check it.

BG