It could be that the ring gear is stripped and likely more than just that. It's probably best to take it apart and have a look and maybe see if you can find a you-tube video. It's a pretty crappy set up so you even want to look at getting replacements.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)