Originally Posted by Imped
First off, you're making the incorrect assumption that more RPM's = hotter operating temperature. What you're not taking into account is the fact that the engine is working harder now than it will be when you've got the proper diff gear ratio installed. Right now, your engine has less leverage on the tires and the towing load due to the higher gear ratio. With 4.88/5.13's, the engine will have to work less because it will have more leverage thanks to the mechanical advantage provided by lower gears. Less work = less heat. So that's a non issue.
As for 4.88 vs. 5.13 when not towing, the difference is 5%.....not much. If 4.88's aren't too low, neither are 5.13's. You'll be sitting at around 2800 RPM @ 65mph with 5.13's vs.~1700 RPM now (assuming you have 3.21 gears). You say you drive in the 70-80mph range......trying to push a brick-shaped vehicle through the air at those speeds will have a more drastic effect on fuel mileage than running @ 3k RPM. Your 3.8 is perfectly happy at those RPM's and truthfully, at those speeds, it needs to be up there in order to efficiently move you. Lugging the engine is much worse than spinning a couple hundred extra RPM's.
Actually, I think you are thinking about it incorrectly. More rpm equals more movement which means more friction and heat. It also means more detonation which means more heat.
If you Jeep isnt geared correctly and you a driving up a hill it will bog down. It wont work harder. It does work harder when you downshift which goes back to my point.