drain the differential and look for excessive wear. if the system is grenading, you'll find lots of shavings in the bottom of the pumpkin. BMW probably equips the units with a magnet, but even if they don't when you drain the oil, you'll see lots of debris, if your system is grinding itself to death.
however, one thing i would check are universal joints. i believe ours are unserviceable, which means they'll just fail. i would check there, and see if any of those are out of spec.
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Gabe
'13 335i M-Sport
Past - '06 330xi, '07 X3 3.0si, '09 328i, '12 X3 XDrive 35i
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