That's something I did also on mine, a pair of Firestone Ride Rite Air Bags in the rear 10 full years ago on my '97, and they were a wise buy.
When I moved here to NM, I easily had 700 lbs of weight in the back, plus 300 lbs of tongue weight from the Cargo Trailer in tow, and never knew it.
Two hours to install, and ten years later they're still holding up well. Of course every so often I get underneath, treat the bags, and lines with a good protectant (no not Armor All junk)
For no loads, I simply keep about 35 PSI in them, they give better stability-handling, and as well the truck has a better stance to boot. No regrets.