I posted something similar once when I looked at a 2500 Mega Cab Laramie Longhorn 4x4 Cummins and it had about 1,700 lbs of payload. I was told by one poster..."put 1700 lbs in the bed of the truck and it won't even squat it." I kind of believe him.
It was one of the facts that have lead me to call the payload sticker the silly yellow sticker as it makes no sense at all.
Ford and the F150 has a GVWR on the Max tow packages that is very close to the Sum of the Axle ratings. While vehicles like the Ram 1500 have a huge difference between the sum of the axle ratings and the GVWR. I didn't compare the difference beteeen the 3/4 ton axle ratings and the GVWR but I believe they are more related to the GVWR of 10,000 lbs they are trying to stay under than the components capability.
Thanks!
Jeremiah