RippSpeed, the clunk you are talking about is another totally different clunk that also happens to me at the same time (rolling 10-15MPH and hitting the gas). I believe 3Hummer is correct in that this problem is related to the transmission downshifting. I personally can't stand it, but have no idea how to fix it.
And just to provide an update to the original issue that prompted me to start this thread, the clunk that I felt through the floor seems to have gone away after the weather dried up. I am waiting for more rainy/humid weather to see if it comes back, at which point I think I would be able to conclude that the clunk is in-fact caused by the body mounts and is directly correlated with rainy weather (and therefore nothing to worry about).
Thanks for all of the responses guys!!