Sounds like your shocks might be bottoming out. If you remove them from both sides you can pick the truck up by the frame and measure the distance between the upper and lower mount point, then stretch the shock all the way out and see if the mount points measurement are greater than those on the truck. Now pick the truck up by the axle and measure that distance, less two inches,(for hitting bumps), collapse the shock all the way and it should measure LESS than the distance on the truck mounting points. If it measures more it will tear off shock brackets, ruin the shock, or both. If your shock doesn't have a dust shield you can clamp a fuel hose clamp onto the stem(lightly) and check after driving, if it's pushed all the way to the top of the stem it's the wrong length.
