Jeep Cherokee to Jeep Cherokee then.
My 85' and 86' got about 20-21mpg @ 65 with 3.73s, and 31's, but it was a pooch. When I went to 4.56s, it dropped to 19-20mpg @ 65, but the available power skyrocketed.
When towing a camper (1,200lb popup) I could never get better than 12mpg with either gearing.
Like I mentioned, I don't think you'll gain anything by putting in an OD gear because it takes the engine out of the power band and into the pooch band. You may gain 1 or possibly 2 mpgs, but because of the aerodynamics of the vehicle, or lack of, the power will suck at highway speed.
Ron