I can't speak for the differentials, but I recently installed an M-B rebuilt transmission and I consider it money well spent. In addition to getting that "factory quality", their rebuilt stuff comes with a two year warranty as well.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles