Apparently the owner's manuals changed for 2005. My 2004 manual recommends 75w140, and I believe all later manuals call for 75w90. I went with Mobil1 80w90 first, then read comments about Royal Purple helping quiet noisy gears, and changed again to RP 75w140, but no change in the whine/ring. Apparently MB changed their minds, so I would recommend staying with the later spec of 75w90 (or 80w90, which I think is more readily available).

Check your rear axles. Apparently slip fit or bolt in (differential side) were used, with the slip fit being in later production. Most documentation says 2004 has bolt in axles, but mine and at least one other member here have slips. (my car was built 11/03) I've seen low mileage diffs on ebay (slip fit) going for as low as $300 shipped. I don't expect bearings to make a difference in mine, and if everything else checks out, I will probably be buying a late model year, low mileage diff myself... This is my daily driver, and I get tired of working on it. If it's down, I have to drive the wife's mini van. My lifted K5 kills me on gas, and I won't take my '67 Belvedere into crazy morning traffic. I see someone get rear ended about every other day.