This rattle has been driving me crazy for a while and I finally found the culprit and fixed it. The non-replaceable nylon bushing in the tailgate hinge has worn enough to render an aggravating rattle over bumps both on and off road. With the tailgate in the upright latched position, I grabbed the bottom of the tailgate and had about a 1/4" play up and down. I was trying to find something tough yet thin enough to take up the space. I saw my wife cutting vegetables on a paper thin cutting sheet and I thought to myself "that would be perfect". If it's tough enough to cut on, it'll be tough enough to use for this project. So when she went out for the evening, I grabbed one of the sheets and cut a 4" long X 1/2" wide piece, took the tailgate off, and rolled that piece into the female part of the hinge that is bolted to the truck. I sprayed some lube into the hinge and reinstalled the tailgate. The movement was gone and so was the noise.