Could it be a bad tranny or transfer case mount? What else is down there that could hit the floor. Also remember that vibrations and thumps can be deceiving. They can travel through metal and seem to be coming from somewhere else. Same goes for bearing/shaft noises. We once spent two days trying to find a bad bearing in a combine at school, only to find that it was one on the other side of the combine from where the noise was coming from. It was half as loud on the other side of the machine.
2000 WJ Grand Cherokee Limited, 4.7l V8, Quadra-drive
Farming is easy when your plow is a pencil and you are a thousand miles from the cornfield. -Dwight D. Eisenhower
Don't curse the farmer with your mouth full.
Ah, cows. A thousand roast beef sandwiches wrapped in a gorgeous leather coat.