Its a bit more complicated, most 3/4t Fords are D60 and 10.5 Sterling, only a 1999-2001 had the 50's before going back to 60.
A D80 is not just a D80, they came in several versions. The Ford dually D80 have larger axle shaft or higher spline count than the SRW Dodge D80.
The Sterling 10.5 has a 9000 lb weight rating. Can't get enough tire under it in most SRW apps to meet that so usually limited to the tire rating in use and the diff isn't large enough for recent really high hp/torque at those high axle loads.
Towing really high loads with 1 tons has been the norm for farmers for 100 years, its just how it works.
Our old 2001 F350 srw 5.4 regularly gets 3000 lbs put in the bed then a 27 ft bumper pull hitched up that is about 15,000 lbs loaded with hay. Its not going anywhere fast but it handles it fine. The neighbours old F350 dually would get hitched to much larger loads and only has a 6.9L diesel. This is not highway speed travel or hotshoting, just moving farm products around.