Ford super duty 3.31 rear end. Will it be a big mistake. ?

It is all about torque. The torque multiplier is accomplished by the differential and transmission. The higher gear ratio of the differential buts more demand on the transmission to produce the torque for high loads. That means the transmission works harder to make up the torque. So in a practical sense, it boils down to how often the truck is required to pull heavy loads vs no loads. A good indication is how often the truck shifts gears to maintain the speed limit with normal driving. Diesel trucks are not as sensitive to torque demands as a gas truck.