Somebody makes air assist bags for these trucks I know. Another option is a set of GM factory overload leaves which were optional on coil spring trucks. Shifting the load reward combined with HD shocks and a load leveler type trailer hitch works well also and won't reduce your unloaded ride quality.
Give these guys a call,
http://www.awdirect.com/ they sell tow truck supplies, including air springs by airlift and firestone.