Sounds like you need to look at the load helper airbags for pickup truck leaf springs
This vehicle weighs about what 5 pickups weighs. The ones Leatherhead found should work out though. These are going between the frame rail and the top of the leaf spring pack. It's for the truck in my avatar. I'm going to pull a few leafs out of the pack as well. The bags are to make up for the reduced capacity and allow to adjust the ride firmness on the fly.