How do you recognize the gears slipping? The car shifts pretty smooth, I'd hate to mistake easy rpm drops as normal when it's actually slipping.
What were your symptoms?

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01 X5 3.0i (gone)
04 X5 4.8is (gone)
06 X5 4.4i (gone)
09 X5 35d (current)
12 X5 35d (current)