There are a couple of places to check carefully and you may fix your suspension for little money. There is a coupler in the engine compartment, just over the steering shaft that goes bad. They will crack, leak and finally burst, with a loud bang. There is a pressure relief valve in the rear, just above the height sensor that is hard to check for leaks.
I had a problem with my Navigator for about a year before I found the valve was my problem. The coupler up front broke with a bang and I got one from Ford for about $28.00, I was checking everywhere in the system for leaks and finally, in desperation, I removed the pressure relief valve and replaced it with a brass "T" that I had made up for use on my Mark VIII. Now with the "T" in place my suspension works as it was designed to and stays level. Before the "T" fix I had to put the truck in 4X4 low and air up the suspension and it would stay up for a couple of weeks before getting soft and starting to sag.
I have a H.D. 26 foot toy hauler and my suspension now allows me use it with no problems. If i had converted my truck to coils it would be a tail dragger with the trailer on it and I would be looking at expensive add-ons to get the truck to be level with the trailer attached.
You have a very good suspension system in your truck and if you ever want to do more than just cruising on the paved roads with no load in or on the truck you should find and fix the problem.
I converted my Mark VIII to coils from a T-Bird SuperCoupe before there were kits available and I am satisfied with it, but not really happy. I never carry much of a load in it so it always rides level. I too wish I had kept at it and found the problem(s) it had and fixed them, if for no other reason than the air suspension is a unique and superior system when compared with coil or leaf spring suspensions. For example; if you go off road and put your truck in 4X4 low the truck will raise up about 1.5 inches for additional ground clearance and if you ever pull a heavy trailer, you hook up with the suspension off, hook up and then turn the suspension back on you can see the truck raise up to normal driving height which is much safer than having it sitting low in the rear with a trailer and it is also unsafe in that position.
In any case it is your truck and you should do whatever you feel is best for you and your family.
Sorry for the long post, but I hope it helps you make your decision.
