I would think that you should be able to have a local spring shop /truck suspension shop rebuild your existing leaf springs, you might even consider possibly adding an additional leaf?
Ask around, find out who is reputable in your area.

 

If you have the capability to do some basic fabrication, your best bet would be to add in a set of air bag helper springs. That truck was not really designed to be a freight hauler. Every spring setup is a compromise anyway- if you add in more spring, it's going to be stiff as hell when unloaded. Even the helper style spring that theoretically only engages when loaded adds stiffness during regular use.

I have a set on my Excursion that occasionally sees some tongue weight on it. The bags make a world of difference. Without them, the truck would be un-driveable with my boat hooked to it.

 

If you want to just go the stock-ish route there are companies out there that offer uprated springs and parabolic springs

 

Parabolic springs are best for off-road. You can simply add larger shackles on the rear springs to raise the ride height.