You need a suspension setup with air bags. Most people currently use them for lowering their vehicles but they have been used in trucks for a long time as a temporary way to increase carrying capacity without all of the harshness in the ride that would be experienced if you just had very strong springs.

It sounds like the best setup for you needs would be to mount the air bags so they assist the leaf springs usually found on a rear axle setup. Contact a professional suspension shop in your area, they should be able to help you out.