I put a set of Firestone Ride Rite airbags on my 1996 F250 a few years ago to help my truck's old springs when towing and hauling heavy stuff. Included in the "heavy stuff" category is an auxiliary 91-gallon diesel tank to keep me running without the need to fill up for days.
I know the leaf springs are getting tired with all the work they are required to do, so I got the air bags to keep the truck level with all loads. The air bags can be filled with just the right amount of air to help support the rear end for the best ride under all circumstances.
An alternative would be to get new leaf springs, but I wanted the extra flexibility that air bags give. I get a better ride as a bonus.