Adjusted the tailgate saddle bracket couple of days ago and no more clunk! Yay! Very easy to do, just loosen the screws and move the plate as required on both the body and the tailgate. Both sides, especially the tailgate side, were pretty loose and had probably become that way after all the bouncing around on the dirt roads. Simple fix, just wish it didn't take so long to find out what was making that noise in the first place!