Questions about tire and axle loading

I agree with Buzzcut . I think you will find that the rear axle rating is based on your spring pack . If you change your tires ( they are close to limit) and add to the spring pack it will not change the official GVWR , but for all practical purposes ( actual performance ) it will . Ford had many options , yours happens to be one of the lower rated .

https://www.fleet.ford.com/truckbbas/topics/2014/14_SD_Pickups_SB_Updates.pdf