The F350 has 3500+ lb rated wheels, upper overload springs and 2" taller rear axle block to give it the higher rating over a F250. Most of these items can be upgraded on F250 by finding one with the correct options (Camper package and wheel upgrade).
The SRW 3/4t and 1t trucks all use full floating rear axles capable of close to 10,000 lbs - You just need wheels and suspension that can handle the desired load within the axle capacity. Research the Sterling 10.5", Dana 80 and AAM 11.5" to see for yourself.
For me to carry a 4000 lb TC on my 3/4t, I needed StableLoads and 3500 lb tires on 18" rims. I added more upgrades due to the heavy trailer I towed behind the camper but never exceeded the axle capacities or the wheels I was using.