True that the differential is the same Dana model on the 2021s. Page 52 shows the rest of the story. The F450 Axles are bigger with more splines and the brakes are much larger with almost double the liner area for the rear with also bigger caliper pistons.
https://madocumentupload.marketingassociates.com/api/Document/GetFile?v1=5641565&v2=111920091727&v3=60&v4=aab797d46fa16d22dd60927744e1b23cba1ecf1722b9a82790d1e244&v5=FalseI think the 9900 RAWR on the F450 is understated. If it is right for the F350 DRW with bigger axles, bigger brakes, and more tire capacity on the F450, I think that is a reasonable conclusion even for the 2021s that the F450 is more capable even though the stated payload is less.
To be clear, you pay for that capability with a rougher ride and more expensive tires, and no ability to air down.