early 4.4s and all 4.6is's have rear air suspension, it was optional on the 3.0s, then the full air suspension came on the 2004+ models (some optional). The air suspension can be reprogrammed by the dealer (however a lot will not do it due to the "potential" harm to suspension components). On the full air suspension x5s there is something called an intrax module that intercepts the signal from the suspension and lowers it even more. But to lower the suspension on your x5 you would have to get new front springs and someone to reprogram the rears.

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options