One simple test is to jack up the car, turn one wheel and see the response is on the opposite side. Same rotation direction? Posi-Trac (Limited slip) opposite? Non Limited Slip (Open Rear). This simple test works on mechanical rears only not electronic or ABS driven/ managed diffs.

Disclaimer I have never tested an X5 diff to see if this works. Some will post observations shortly I am sure.

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green