I guess that would be my final option after replacing the shocks/coils if I still had issues. Having to install the compressor to power the bags adds a lot of cost and complication to installing the Air Lift bags. I'm not really exceeding the load limits of the standard suspension so I'm hoping I don't need bags.
I guess I am not truly understanding what you are trying to accomplish. Based on the various posts you have made in this conversation, it sounds like you want to have a vehicle that maintains the stock ride height under no load, but doesn't sag when you load it, or at least sag too much. I don't know what "too much" is for you.
Springs are a pretty simple device. They give under a certain amount of load and will give more under more load. They have what's called a spring constant. For every 100 lbs of load, the spring will deflect a certain amount. The less deflection you want, the stiffer the spring you need, if you are only relying on the spring. You can achieve almost negligible sag with very stiff springs, but you will have a very rough ride with no load.
The way I see it you have 4 options:
1) Don't do a thing, keep your existing springs and just live with the amount of sag you get when you load up the vehicle with 500 lbs of stuff in the back.
2) Go with a Variable rate spring so once you get to a certain amount of sag, the stiffer portion of the coils kick in and slow the rate of sag, but it will still sag some.
3) Go with really stiff springs and live with the stiff ride all the time you are not loaded down
4) Get some sort of helper bag system. There are a number of budget friendly options out there and not all require you to purchase and install an air compressor. Many guys have installed helper bags and just run the air lines to a valve near each wheel or out the rear near the license plate that you can manually inflate or deflate the bags like a car tire.
