If you switched trannys and had an auto, would that make any difference?
Would the machine be fine on flatlands but when you get to a slight-moderate grades you'd notice the lack of power, downshifting, slowing down...
And then if you were pulling a trailer it would make matters much worse...?
Is the motor a 7.3 or newer 6.0 turbo diesel that is the same as the standard ford passenger truck motor...?
So it can handle the weight, it's just that it doesn't have the power to pull it up moderate hills and a bigger motor would be better?
"I have marveled often at the thin line that divides success from failure and the sudden turn that leads from apparently certain disaster to comparative safety." Ernest Shackleton, Antarctic Explorer, Sea and Land, 1874-1922.