There are Add-a-leaf kits, and spring overload kits available at your local part store. I've used/installed these kits on smaller trucks I've owned in the past. They're fairly inexpensive, work well, and install in an afternoon. Air shocks are another option. Back in the construction days, I drove an '81 Ford van with air assist. When we had a lot of weight on her, just pump it up! Empty? bleed 'em out!