If you put air bags on the truck in addition to the factory springs they increase your spring rate loaded or empty and will make your ride harsher. As some have noted before if you use them to lift your truck off of the overloads or better yet remove them and the airbags will probably add less harshness than the steel springs did and give a net improvement in ride. At one time my father and I have very similar trucks and loads. I had the factory overloads and he opted for airbags. I thought that his truck rode much better than mine when loaded. One thing that I have not heard of in a pickup but came as standard equipment on my Freightliner is ping tanks. They are simply tanks in the air line going to the spring and are used to absorb some of the air from the air springs when you hit a bump, thus reducing the spring rate of the airbag and softening the ride.
I am really curious about those Sulastic shackles, I may have to try them on my new truck to help the empty ride.