Forget about the truck's payload rating as it is based on the configuration of the truck as it left the factory and subtracts 150 lbs. for every passenger seat in the cab. That is all that the manufacturer can do. But you can change the springs and change the tires with little effort and expense and increase the safe payload capacity.
The bottom line in terms of payload capacity is the load capacity of the rear axle and wheel bearings and the one in your truck is rated at 10,900 lbs. so it is easy to see that the true load limitation is the rims and tires of the truck.