Most of the rating difference is in the clutch. Ford doesn't want to warranty your clutch while you learn to drive.
We all kick ass at driving with a clutch, so they shouldn't worry about it.
But, case in point, my skoolie is a 1991 International with an Eaton 6-speed. I bought it with only 67,000 miles on it and it had a new clutch and new driveshaft joints. Hmmm. All of my bus's sisters for sale at the same time were automatics with 300,000 miles on them. It's a fair bet those Allisons were original.
This bus comparison is a good one for this tow rating discussion. The housewife bus drivers destroyed a clutch in a 20,000# medium duty truck which is rated at 60,000# GCWR and they were unable to destroy the automatic trans vehicles.
If you want to use the automatic tow rating for your manual Ranger, you need to follow 2 rules. One is to get the clutch up quickly when you first get moving. A stock Ranger clutch isn't that good for this, but load some torque on the motor and get the pedal up. After that, the clutch really should not see any more heat until the next time you have to stop. You never need to slip it between gears. Because the second rule is to never let the clutch out when there is a speed difference between the engine and tranny. On a school bus a speed difference means all the crap in the bus goes flying and all of those drive shafts linked together under there whip around like a python being tazed. On a Ranger with a load, the clutch just smokes. You have to get used to not applying any gas until you have the clutch up. You have to find those spots for yourself. Mostly, make sure you have at least 4,000 on the tach before you shift when you are pulling. You don't want to have to use the clutch to pull the tranny up to speed after a shift. You should shift, let the clutch up and then feed in the power. On downshift, you have to kick the throttle before you let the clutch up. You have to get a feel for what your downshift points are. It's less important to get the downshifts correct on a Ranger because the motor doesn't make much negative power--mostly try not to overspeed the motor.
A clutch should die of old age, not wear.
But I should say--I prefer automatics. I've got a lot of experience on heavy stuff as well as lighter 1-ton and below. I don't really care all that much, but given the choice, I'd rather just drop it in GO and let the truck worry about it.