I have a 2009 Ram 1500 and I get a rattle also seeming to come from the right passenger side of my truck bed. Mine seems to show up when the weather warms up. I'm thinking it's because things get loosened up. Mine started over a year ago when I took it off road (and caused other damage also).
I'm with you, I can't seem to figure out where it's coming from either.
It could be the rear sway bar bushings (not the links at the end, the ones that hold the bar in place) binding up. you can try cleaning any rust around the bushing and spraying a lubricant into the bushing itself.
My truck had a rattling sound when I would go over bumps especially speed bumps. It took a while before I found out what the problem was and it was where the exhaust is held down with a rubber to the chassis and it was loose and every time I would hit a bump the exhaust would hit the under side of the truck.