Matt, that sounds extremely logical and -- so far -- the most likely cause of the clunk. It's funny that you say body mounts because one of the possibilities that I came up with last night while I was driving was body mounts (maybe the whole cab was moving on the frame), but then I thought "how could that be?". It would also make sense that I felt it for the first time last night because it has been raining hard up here in the Northeast for the whole week and last night was the first time I drove the H2 since Monday (when it was dry out), so the body mounts might have been wet and more prone to "slip". I will see if it stops today or tomorrow now that we have dry weather here. If it does stop "clunking", then I think this comes close to proving that it is the body mounts.
Thanks again, and if anybody else disagrees and/or has any other ideas, I would certainly still welcome them.
Sun-