you mean air springs. basically a supplemental rubber bladder that get installed inside the rear coil and under heavy load you can air up to compensate for the weight.
Firestone and Air-lift are the way to go.
If you want a replacement shock that can help support a load, I personally do not think it is a good idea since the shock mounts on a jeep were never designed to support any weight, but I ran a set of shocks with a coil spring on the rear of my pickup years back to help with overloads. I got them from JC whitney and they were moderately effective at best. I ended up going with Air-lift set up.