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They make rear shocks that are load adjusting...basically they have springs around the piston instead of a boot. Another option would be factory helper leaf springs. Lots of options there.
I have air assist rear shocks, I can change the PSI anytime to match what I have in the back/towing. Like BLD said helper springs will help much more that any shock will regardless of style (coil over shock or air adjustable), they make them in a few different styles, some of which may raise the back slightly but they also have ones that sit lower so they don't "help" until you get a certain weight on the back end of the truck.
You can also get air bags They sit on top of the spring and bolt to the frame. Add some air when you're loaded and they pick up the extra weight.
Use to run air shocks on my muscle cars way back when, to pick up the back but I think they put a lot of stress on the shock mounts. They're OK though.
I think I've got a pic here of helper springs (usually called overloader springs). I'll see if I can dig one up and attach it here. If you get the factory style you should already have the 2 little groups of 4 holes in your frame to attach them.
I feel like helper springs would lift the rear a little bit though. At least brand new ones would. And I've thought about air bags but I hear very mixed reviews about them. I'll look farther into your options though
I run air shocks as well. 35psi keeps it at stock ride height and if I hook up the travel trailer, I'll pump them up to 100psi and it level the truck out. Work great for me. I have overload coil over shocks on my Silverado. Raised the back end 2". Ride fairly rough. Love my airshocks.
