Originally Posted by xJimbos

Oh and road salts can make stains like that too.
To prevent salt stains just spray a hose on the underside of your truck every once in a while. Don't spray too much around the top of the rear diff. (you could kill your veh. speed sensor like I did) and don't spray too much around your engine area, esp. if the engine is still warm. Metal does not like rapid temperature changes.