85-140 is way to thick and not what is recommended by MB. First try just changing your old oil and see what happens. Your diff could be whining because the oil is old, or you may not even have enough in there. I changed the diff oil in my '84 300D about 7 months ago and I swear it was the original stuff in there. There wasn't enough in there, and the stuff that did come out was unbelievably dirty and foul smelling. Just putting in new oil made differential whine go away almost completely.
Just try changing it first.