Did they check the Guibos? How about the driveshaft support bearing? Any sign of decayed or cracked rubber, these should be replaced...even if that's not the issue.

I'd also want to closely inspect all axle shafts. A torn boot could give a hint at a failure, but the joint can go bad without tearing the boot, (just as, if caught early enough, a torn boot can be replaced if the joint is good).

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp