2008 sierra bed is moving like crazy

[bakerboy81]

I have a 2008 sierra. When I bought it from the Chevy dealer they told me that the truck was GM certified but the truck was lowered. So now that I have had the for about 4 months the bed of the truck moves keep crazy every time I go over a little pot hole in the road or train tracks. I have a feeling that its the leaf springs but I'm not sure. All the bolts to the bed are tight. Can someone help.? Thanks