rear end ratio

Suggest the OP save money for the next truck and drive with OD locked out for now. Biggest problem with Ford 4 sp truck transmissions IMO is that when more torque is needed while driving in OD the torque converter unlocks first; then if that is not enough, it downshifts to 3rd. While the converter is unlocked much heat is generated, the enemy of transmissions. Changing to a numerically higher ratio might just run this way more often instead of shifting down to 3rd. Also, likely the OP has a 4R100 transmission which is not famous for going over 100K mi. My son has a 2003 F-150 5.4 4R100—his went out at 100K and he immediately found 3 friends with the same story. Torque peak for the 2V 5.4 is 2500 RPM so running in 3rd (OD locked out) at close to this RPM would give lots of torque and best engine efficiency. Fuel consumption is largely a factor of how much work is being done and engine efficiency. Towing down the road at the same speed is going to use a very similar amount of fuel regardless to the gear. I checked fuel economy towing with our 5.4 Expedition with and without OD locked out—no statistical difference. OP, when you go looking to trade to a newer truck try to get one with 6 or more speeds in the transmission—the closer gears are just wonderful for towing.