I wouldn’t say 3.91 gears are severe off road gears.I would say a truck specd like that would be geared more to severe off road duty, such as logging or oil field work. Not really needed for super B's or what most normal farmer would put it through. Those nut bars on Ice Road Truckers would be a good candidate for a truck like that!! lol
It’s old school thinking that you need 46s to pull super B. I don’t know one truck company that has broken a 40 rear end pulling grain trailers or hauling hay.
Assuming your back survives the suspension.