Sway bar end links. Mine were replaced under warranty. The part in question is the one in this picture, just to the right of my right rear shock, that has the white sticker on it.
Same problem as well. When I shake the tailgate, I can hear it, but not as pronounced. The latches on the ends are part of the problem, but I think the main problem is from the sway bar endlinks or the leaf springs.
I replaced the left endlink when this started on my 02. When the noise came back on the right end link, I just cut a piece of rubber in a circle and drilled a hole in it and created a bushing between the bar and the end link and tightened it down. The metal on metal between the endlink bolt and the bar wears down. At least that was my problem.
My bar on my'04 was the bushings where the bar connects to the rear axle..I installed grease zerts and pumped grease in and squeek stoped..never had any noise from the end links..
When my end links had a problem, they sounded like a rattling or clunking sound when I went over bumps. The swaybar bushings that attach to the axle is where the squeaking on mine was coming from. I pulled off the bushing brackets and greased them, and that worked fine for about 6 months and they started again. I replaced the bushing brackets with greasable ones and have not had a problem in 2 years now...
I think it was Todd Z. who was selling greasable bushing brackets. Might check it out. They do work well!