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A little more info about your truck, like what year, which trans it has etc might get you some more answers. I have a manual trans and run in 6th gear about 1850 at 60 mph.
Theres nothin wrong with towing in OD. Ive always towed in OD, well Tow/Haul mode, which his truck doesnt have. Tow/Haul doesnt lockout OD, it alters the shift strategy and line pressure. There is an OD lockout on mine, but I never use it. Only place I could see locking out OD is in mountainous terrain, lots of up and down. On the flat, theres no point