Given the low failure rate of these, why not get a diff from a junkyard?

Also, given the symptoms, I would want to take a close look at the intermediate driveshaft bearing, (in the middle of the rear driveshaft). Those DO have a high failure rate, and can cause similar symptoms. If there are any tears or cracks in the rubber ring, replace this before considering a diff. Take a look at the guibos as well.

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