Another possibility is frame flex. I know my truck gets into an annoying vibration rhythm at certain speeds on a highway that has expansion joints, and there is a similar chassis vibration when I hit bumps/potholes at other speeds.

I used to think it was something I could tune out with better shocks, but I figured out the actual problem one time when I had the camper shell mounted a little too close to the cab- the chassis vibration made the shell contact the high-mount stop light on the cab repeatedly, in sych with the vibration. Its kind of annoying. Not sure what the best way to fix it would be...