on some of the sport haulers i see that are often driven empty around town after towing a race car or horse trailer to an event, there is a hidden block of weights simply to keep the rear "heavy duty" suspension somewhat loaded.
i have often thought about mounting some concrete filled lolli columns inside my rear bumper, just to have an extra ~150lbs behind the axle to keep things smooth. my OBS 350 gets kinda jumpy when both tanks are nearing empty and there is no load in the bed.
the extra weight trick wont help up front, as its loaded pretty well with an awesome engine