To the OP:
The only things you need to do to increase your payload on the 2500HD GM D/A 2003 CC SB truck is replace the OEM tires with 265/75/16 "E" good tires which have a 3415 lb capacity each at 80 psi VS your OEM 245/75/16 "E" tires having 3042 lbs capacity each at 80 psi. I personally only use Michelin LTX and they are fabulous on our 2004.5 Chevy 2500HD D/A CC LB 4X4 truck and are silent running with very good traction in snow and are high mileage tires. An additional 373 lbs capacity on each wheel.
Then replace the OEM shocks with a higher capacity shock. I'm very partial to Rancho 9000X adjustable shocks and they work simply great. Got over 140,000 miles on mine a still are working fine.
Next, I'd put airbags on as they are truely and fully adjustable to match the load you carry or are not carrying. Other types of spring, rubber, etc addons are NOT adjustable and give a very rough jolting ride when only half loaded or lightly loaded. Had them and know all too well!
Basically, these things make your 2500HD the same as a 3500HD SRW GM truck. GM only makes ONE HD chassis for the 2500, 3500 SRW, and 3500 DRW HD trucks and just changing these items and add's 2 more wheels on the rear plus the fender hips box sides for the DRW version. Identical other than that for your 2003 truck model except for stickon badging etc.
Go down to a dealer parts counter and ask for the rear axle actual dia for each of the 3 trucks for a 2003 model year and you'll find out for yourself. The DRW axle only has a different plate welded on the ends for DRW mounting. The AAM axle has 10,912 lb rating for the actaul axle used. The 6084 lb rear rating from GM is because the 2 rear tire having 3042 lbs capacity each totals 6084. It's the limiting and governing component. 265's give 6830 lbs capacity at 80 psi or an additional 746 lbs on the rear alone.
