my truck does this too, it feels like its coming from the rear end, so my guess was the u-joits. i ordered two new ones, havent got a chance to replace them yet... also, when i'm stopped at a light and go to take off, it feels like the drevveshaft spins a bit first, then it clicks on and taks off... it feels like a THUD and i dunno what's causing it... is my rear-end on the way out or what?