Suspension ID and advice

I bought my 2011 TX Pro used and would like some help and suggestions with the suspension setup. It looks like it is lifted with a Toytec spacer in the front and an additional leading he rear, with stock bilstein struts. The added rear leaf is nice when I have my snowmachine or four wheeler in the bed but is super stiff when the bed is empty. Stiff enough that the back end can be unstable when hitting large bumps on dirt roads.

What setup should I be looking into to maintain the ability to carry a 500lb load in the bed without excessive sag but still maintain the relative comfort when empty?

 

Airbags and springs that work unloaded. Any spring that works at 500# in the bed is going to be harsh as hell with the truck empty. And any spring that is good empty will be very saggy with 500# on it.

I run Dakars, which suck empty, are acceptable with my 200# tool box in the truck, great with about 400# of gear, and acceptable with 1000#s of stuff.

Alternatively to airbags, get a 200# weight to haul around when you don't have a sled or quad in the truck.