'04 silverado crew cab bed shakes?

I have a '04 Silverado 1500 crew cab short bed, and the bed shakes/ wobbles up and down when I go over bumps in the road, especially over speed bumps, it doesn't do it as much when I carry a load. I don't know if it's the shocks or any mounts,frame etc... What could it be? please help it's getting annoying. Thanks