If you're going into uncharted territory, here's a few links that *might* assist.

Performance Final Drives
Performance Gearing - are you ready for the ride of your life?
BMW X5 - NV125 : Automatic, Standard Transmission and Transfer Case Rebuild Kits, Cobra Transmission Parts 1-800-293-1848
NV125 Transfer Case, BMW X5 Parts
diffsonline.com - Custom Geared BMW LSD Differentials
Wavetrac® Differential - Application List
Sideshafts - Passenger Cars - GKN Aftermarkets & Services

If you purchase an E70 3.0Si, I wouldn't be opposed to trading differentials.

Best of luck in your search man!

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Previously owned: '03 4.6iS Dinan Supercharged