Hmm, I've been reading this thread, and the above sounds somewhat like what I've got going on. It only "klunks" when I pick my foot up off the gas from accelerating, and as it shifts, KLUNK. It started after a trip to the mountains, with a SMALL trailer load, and some brutal climbs for my old truck. I originally thought it was the tranny, but it shifts like new when driving, and accelerating. I've been under it several times yanking on stuff, and I can't find anything loose.