If you don't want to exceed any limits (other than the manufacturer's axle or GVW posted on the door) you could look at your tires and see what they are rated, then choose not to exceed you axle rating based on them. If you have rated tires above "E", then you could conceivably go up to your wheel/tire limits, or even up to the axle itself (the same 11. 5" AAM is rated at 12. 5K in other applications. As well as being the same axle in the 3500 DRW with different shafts to accommodate the DRW). As far as your truck chassis is concerned, you are driving an identical truck to the 3500 DRW with the exception of the wheels and hubs and (rear) springs. If you have airbags or helper springs in the rear, and high capacity tires and wheels (19. 5's), you will be perfectly safe running a higher weight (up to the 3500 DRW rating or whatever the Wheel/tire maximum is. . 9000lbs with mine).
As far as the guys that say don't ever run above the manufacturer weight on the door... good advice. But then again, that is there for insurance and warrantee reasons, and has very little to do with what your truck is capable of handling safely if you have taken the proper precautions. You just have to be prepared to accept the risk of your own venture into the unknown (running higher than recommended gross weight).
The most important thing to remember is the advice you receive on this forum is opinion. Most of the opinions, especially the ones that give insurance and legal advice on your risks and liabilities, are unsubstantiated and based on rumor and personal opinion, not fact. I have not see one instance of a qualified licensed litigator quote any law, or case, to back up the claims of liability or exposure regarding failure to follow the manufacturer guidelines as posted on your truck. Many of these “over gross weight” (OGW) nay-sayers are all for other modifications to the vehicle like bombing, hitches that are not specifically approved by the manufacturer, or even radical truck modification like the “Mega Cab Long Bed”. In every single one of these cases, you have changes or modifications to the truck that could effectively alter the safety of the vehicle. Even so, I have never heard a peep out of the OGW “experts” regarding the legality, or even safety of these modifications.
In pretty much every case, you are at the mercy of your individual state regarding the legality of your operations, and must make up your own mind about what is legal and or safe.