Yeah I see your point about OD. It's true that towing in OD should be avoided, in which case 3.08s would be better for going down the highway than 3.73s. But the other thing to consider is starting out when you're in 1st and accelerating through 2nd. With 3.08s, the torque (and therefore stress) on the transmission while you're accelerating (keep in mind climbing a hill at constant speed is the same as accelerating) is about 21% (3.73-3.08 / 3.08) greater than with 3.73. This is what will cause the transmission to overheat.
As far as towing with a manual, Ford underrates the manual transmission tow capacity to avoid excessive clutch replacements under warranty due to people who aren't very good with a clutch trying to tow with one. In and of itself, the manual transmission is just a set of cast iron gears that mesh and transmit power through the transmission. In this sense it is much stronger than the multiple clutches and bands in an automatic that basically use friction (which means a lot more heat) to transmit power through the trans.
Towing with a manual you'd definately want 3.73 or 4.10 rear gears because that it what will make the difference between burning your clutch trying to get started with the trailer and being able to engage the clutch without revving the engine and burning the clutch.
A ranger with a manual transmission, 3.73 or 4.10 rear gear, a good quality clutch, and good driver (knows how to work a clutch) will be more reliable pulling large trailers than an auto.
But I agree. From the driver's standpoint, it's definately much easier to tow with an automatic.