Technically suspension mods do change ratings just how getting a lower rated tire than stock is suppose to lower your rating.
Take a 2013 2500 diesel. Besides the rear suspension, it is essentially identical to the 3500 SRW axles trucks and all. Take out the rear leafs on the 2500 and put in the same springs that the 3500 uses and technically it should change its ratings. Same goes with putting in an add a leaf or airbags to help the rear suspension handle more weight.
I think people put too much stock into that door sticker. The law requiring manufacturers to add those stickers was due to the Ford Explorer/Firestone tire fiasco back in that late 90s and there is no law in the books requiring the consumer to abide by that number. It was meant as tire inflation and loading information for the consumer and can change if you add tires with a different load rating. Sometimes that number is actually limited by the tires on the vehicle, and some times it is limited by other things such as max GVWR limits of the trucks class.
For some trucks that don't come close to their truck classes max GVWR rating, like 1/2 tons, it is probably best to stick to the load ratings because it is not max GVWR that is the limiting factor. For trucks in classes like some class 1 or most 2b trucks that are limited due to the max rating, then I would take the ratings with a grain of salt. I am not saying go out and put 4,500 lbs in a 2500 diesel, but I would say that it could easily handle at least 3,000 lbs which is well over its 2,000 lbs rating.
