Not to put a damper on the tank but you should check your truck weight at a truck scale sounds like you may be overweight on the rear axle. No, adding air bags and helper springs will not increase rear axle carrying capacity and you will find out the hard way if pulled over by the DOT's. You'll be recieving a healthy xmas present, you may not like. There will be a lot of posters who will tell you that they carry 10,000 lb payloads + in a half ton, like I've said weigh your truck and be informed. This is for your info do what you may.
2 sleds 650+/_ 1300lb
fuel 700lb
buddy 200lb
gear 200lb
Like sex with someone other than your wife, young girls etc. all good till you get caught, then it hits the fan.