New trucks have way higher payload ratings. Yet all along, they’ve had the same parts.
Look at a 20 year old Dodge 2500, manual trans.
It’s got a Dana 80 rear axle, rated for 11,000 pounds by Dana. Same one the same era F450 had.
The payload rating sticker said you could barely haul a load of wet leaves.
Dodge didn’t even sell a single rear wheel badged as a “3500” then, because it was redundant when the 2500 was literally the same thing.
It really comes down to wheels and tires.
The hard parts in 2500-3500 trucks are all similar. They put stiffer springs in some. They put taller suspension on some. Ford puts a couple-inch taller block between the axle and springs, and slightly stiffer springs into its SRW F250 and calls it an F350.
Oh, and a bigger number on the sticker.
The all-mighty payload sticker... it’s a magic thing.