[Big Dave]

Think about what you are trying to do.

The OEMs have engineering resources you cannot even imagine. You can bet the farm they've taken a long hard look at this and rejected it unless it uses a direct injection engine and/or hybrid drive. You run into control issues and a loss of torque that makes the vehicle a burden to drive. The OEMs rejected it for good reasons.

Another very doable and proven effective strategy (maybe long term for you) is to ditch the automatic and get a stick. That's a slam-dunk 2 MPG improvement. there are all sorts of kits available to convert these vehicles to a NP4500 five-speed manual. The manual enables you to run a lower ration final drive. Yukon makes a 3.23 R&P set for GM 12 bolts front and rear. That's a 14% reduction in engine RPM at a given road speed If you need more torque, push the pedal and yank the stick and Viola! There it is.

A NP4500 and 3.23 gears. We're talking a 4 MPG improvement and a big improvement in reliability. You keep your 4x4 for off-road driving, and don't have to do any body work.

Not cheap, but entirely doable and you can expect a 33% increase in gas mileage.