Actually, my wagon gets pretty decent mileage, and I did the ignition upgrade (plugs, wires, coil with 12 volts, HEI, dist, etc) and the Motorcraft 2150 carb. Now I'm only in the mid 20's for fuel, but that's not bad.
But don't let that worry you, because There are other transfer cases that came equipped on jeeps that will mate up with an eagle. some need a change in the yokes (or something like that) but otherwise, will work/fit.
Also, earlier jeep trannies (something like 80's thru till early - or mid 90's) will fit the eagle engine. If it has a 4.0 straight six engine, they should (I'm not 100% sure) mate up and fit.
That means more t-cases to choose from. Might have to have a drive shaft modded, or a yoke, but that's probably easier to have done then trying to get a NP-128 or a NP-129 rebuilt like casper says - They're getting expensive/hard to find.
so you could put in a 4 speed auto with OD. That's what I'm looking at doing.
And depending on what you want to do with the motor, you could get alot more fuel mileage.
Some simple upgrades (better carb, HEI, ignition, etc) some more moderate upgrades (cam, 4.0 head, headers), swap to a 4.0 HO motor, or do a stroker.
I guess the point is - I don't think it's a stretch to own an Eagle that gets 30+ miles to the gallon.