It's fun to break new ground. I applaud those that take the risk.

Based on your first post your goal is to get away from the stock automatic transmission you are having problems with and perhaps improve economy.

Going to 2wd will gain some HP/TQ lost in AWD hopefully resulting in better economy. And, a manual transmission driveline fully locks where the torque convertor always allows some slip. Also, to best of my knowledge it would be one of a kind if that is important.

Even though you already have a manual 6 speed, there are so many unknowns in a conversion and still considerable cost and time investment, I would have the transmission rebuilt, add Dinan transmission software and be done with it. I am at about 117,000 miles on original transmission in 4.6 that is heavily modded. I have the Dinan transmission software and have changed the fluid with ZF fluid every 40,000 miles. I recently replaced the internal wiring harness--that's it. The Dinan software will give you more attributes of a manual transmission and you can add more fun if you do a paddleshift steering wheel conversion.

If you want to improve economy I would first try tires that were at least an inch taller to see what happens to economy and if you like how it is on the highway. If that gives you what you want then find diffs that get that same highway rpm with stock tires. The reason to try tires first is the change might not improve economy if the engine is not running as efficiently as much of the time. Rather than seeking a particular highway rpm I would try to get as close to the rpms where the engine is most efficient at the speed I normally travel.

FYI, due to frontal area and weight, the maximum speed of a stock 4.6 is 153 mph. Wind resistance is then a wall.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas