I'd check and see if the shocks are low on fluid. By vibration do you mean the truck starts bouncing uncontrollably when you hit the bump? To test it I would unhook the shock link going down to the springs and move the arm up and down to try and feel resistance. If they are covered in grease and you haven't touched them they might be leaking. You can fill them up with hydraulic oil until the level reaches the fill hole but they might leak out again. (I'm assuming you have the lever action shocks)
