Why do you want to trade out the transmission? And what exactly are you trying to accomplish?
If gas mileage is the issue, you simply will not achieve the cost/benefit ratio to justify a new transmission. If you are just buying it for a normal vehicle and don't plan to off-road much, maybe changing over to a higher gear in the differentials would make more sense. You might get 3 or 4 MPG by going to a 3.73 and that's probably being very generous.
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