He didn't ask how to upgrade the GVWR, he asked how to upgrade the payload capacity.
I'd find out what else was different besides the axle gears between the 7,200 and 8,200 models. If it's the exact same axle and brakes the difference may just be in higher rated tires. I hope you at least have LT tires on it and not P tires.
Air bags will help if the rear is sagging. Bigger better brakes are always a good idea. That's about all there is to upgrade for no more capacity then you need.
Otherwise, find a nice 2500 sub.
