I had the intermediate carrier bearing go on the driveshaft, Depending on where I sat, the noise came from all over when it was really right under the middle of the rear seat. I do most of the work myself, I had this one done because of the stories on this site, replacing transmission seals etc.
The mechanic I use said (and I quote) "this sucked" about $600.00.

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John
2001 ML 320
1988 560 SL
2009 Toyota Tundra TRD
1992 400 E (sold '07) (bought back, '13)
2008 ML 350