The reason they recommend against using OD when towing, would be internal lubrication of the transmissions bearing. The transmission is working it's hardest/hotest at a lower speed and may not be getting enough ATF to the bearings. This was a common failure on certain other manual transmissions when towing. Also the engine is working harder/hotter at a lower RPM, which would in some cases offer the possibility of overheating. Last time I hauled my boat, I found the truck wasn't much thirstier spinning in 4th towing than driving in 5th.
"first gen rangers are 1983-1988
second gen rangers are 1989-1992
third gen rangers are 1993-2000.
first gen rangers are NOT 1983-1997."
My mistake, Sorry.
2009 Ranger Sport 4.0L 5 Spd 3.55:1 Bone Stock
2013 Scion FR-S 2.0L/H-4, 6spd, 4.11:1 Fast Enough Stock