With most trucks the 1-ton is the same as the 3/4 ton but for the extra leaf pack and different tires, including the option for dual rear tires. With GM trucks the only other difference is that the rear rotors on the 1-ton DRW trucks are 1/4" larger so a total 4-rotor difference of less than half of one percent.
The payload rating is calculated at the factory based on the weakest link in how the truck is configured. The more passengers the cab can hold the lower the payload rating by 150 lbs. per seat. The other two factors are the rear leaf springs and the rear tires. Both the springs and tires (and rims if necessary) can be augmented or changed to increase the load the truck can safely handle.