I NEVER tow in O/D unless I am on the flats doing 70-75mph! I dont know about you but I have 3.73 gears and I cant be in OD on hills cause my temps sky rocket(at least 1200-1250) and the "Coal Really Roles"! But when I pull it out of OD the temps usually drop to 800-900. Anyway I will not hurt the truck to run it with a load out of OD. The only I time I found that being out of OD hurt is when I tried to speed(75+) up a hill with 16,000# and I was pushing the motors redline and it over heated but other than that time it has NEVER had a problem.
sorry for the long post.
~Jim