olcoacher, try this. Crawl under the truck and using your arms and/or legs, move the truck up and down as you watch the lower shock mounts (specifically the rubber grommet where the shock meets the hanger on the axle). Do you see the shock moving in relation to the hanger? I bet you will see the movement there. If so, you can try tightening the bolts to "spot torque" (ie, tighten the bolt till you see spots). But, if that doesn't help, then you will probably need to get 4 fender washers and undo the bottom bolt on each of the shocks and slide a fender washer in on each side between the shock and the hanger and then tighten it back down. THe problem is that somehow all the manufacturers for shocks have made the metal tube for inside the rubber mount about 1/8" too short. THis results in the hanger not being able to tighten down sufficiently to grab on to the metal tube. THis results in the metal tube becoming oblonged over time.
I am placing money that you changed out your rear shocks about 5-10K miles ago.