JCarrier,
My 05 did the same thing and turned out to be the lower control arms. They were not torqued properly from the factory (where they bolt to the axle) and would only go "clunk" when I was backing down the boat ramp to launch my airboat. Took the dealer about an hour to figure out what was going on. R&R'ed the lower control arms, torqued properly and everything id fine. . Hope this helps. .