Don't know why you focus so much on why the OP lifted his truck, or why clearance between the diff & ground are an issue. The OP was just giving a description of his rig for anyone willing to assist with info on what he wants to do with airbags. FWIW, additional clearance between the diff and ground can be achieved with larger tires, which is what most do when they lift their vehicles.REGARDING THE INITIAL QUESTION:
First of all, I do not know what a "Zone Lift" is ( or, for that matter, why anyone would want their truck higher than stock. My puzzlement arises from the fact that most trucks I have seen have differential "pumpkins" in the center of their axles. Those big bulges hang down, leaving only a few inches of clearance between the bottom of the differential case, and the ground. So no matter how high you mount the truck's body and chassis, unless you have some way of eliminating the bulge of the differential, you havnt gained ANYTHING in the way of rough road clearance.
As I noted elsewhere - seems like I am not the only one who is not happy with the absurd overly dramatic "forward rake" of a bone stock 3500.
I suspect I am typical of many owner/operators - I only tow trailers weighing in the 10-15,000 lb range...with a 1,000 to 1,500 lb. "bumper-hitch" tongue load, and, on occasion, do use the truck as a back-up passenger car.
Given the above, I suggest rear air bags are an ideal solution. Gives you all the load capacity you could ever need, coupled with a much smoother ride.
I am assuming, without knowing for certain, that the FIRESTONE and AIR LIFT systems are similar. Either permits us to remove a leaf from the rear suspension, making the truck ride ALMOST level when empty, and completely level when loaded .
By running maybe 10-15 lbs air pressure in the air bags, they serve as a cushion, giving the truck a much smoother, more controllable, safer ride.
The air bags restore any load-carrying capacity lost by removing a rear spring leaf.