front and rear diffs are interchangeable from 4.4 you will have to do both. The diff change would net you a 400rpm change. From 3600 RPM to 3200 RPM at 75mph. Then a 6 speed swap would help you go from the final gear ratio of 1.00 to 0.86 which with the 3.64 gears would drop your 75mph RPM down to 2750 RPM.

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2002 BMW X5 4.6is soon to be LSX swapped. Silver on Imola

2003 BMW X5 3.0i 5 Speed Manual, Sport Pkg, 190,xxx miles and counting. Black/ Black 20" 4.6is Wheels, OEM Stainless Steps, ZHP Shift Knob