Worn rear position upper control arm bushings for me.

Replacing the rear position upper arms has allowed me to get camber back to spec on 2 different E53s that had excess negative rear camber.

Comparing old and new, the bushings seem to want to deflect inward over time, thus causing the top of the rear hub to tilt inward and give you negative camber.

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04 X5 3.0i auto
03 X5 4.6is