Several things to check.
1) shock could be bad and causing the clunk (I kind of doubt it though)
2) There have been some problems with shock bolts that were not tight enough. Crawl under there and make sure the bottom bolt is real tight.
3) Sometimes the spotwelds on the body behind the fender can break letting the panels move around and make noises like this.
4) Fender liners, battery cables, and non-stock air filters can all be possible sources of noise.
Just be thorough and take your time to examine everything on that corner of the truck, you'll find it.
Good luck,
Mike