If by "Air Shocks" you are talking about replacing your shocks, then this does nothing to help your payload.
What you want is "Air Bags". But I wouldnt just replace your coil springs cause its never as easy as just that. You could do that but IMO it wouldnt be a good setup.
You got the three routes you could go with this, replace your coils and use the old trailing arm setup, which was never good for anything really. Cheap way to go but not the best.
Add helper bags. Cheaper than replacing your coils with bags, but with the coil setup would take fabrication still. Not a bad option, I would have to say better than replacing your coils with bags.
The way I would go, a 4 link with a notch and bags. More expensive the the other two ways, but IMO the best. And while you are at it you could bag the front too for a just a little more since you would already be paying for the most expensive stuff already.
It is a sepndy way to go but well worth it IMO. You can lay out your ride, drive it as low as you want, and air it up for those pesky speed bumps, plus it will haul more of a load then your coil setup ever will.
