When was the last time the differential was drained and filled with fluid? Is there any evidence of leakage?
I agree with the suggestion that you plan on swapping out yours for another as once metal parts are damaged they do not "heal" but you might try changing the fluid to help damp out the resonances in the bearings or gears.
These noises in bearings usually arise when the surface of the rolling elements and the races become uneven enough for the load conditions to allow metal to metal collisions, and/or ball skidding. Both of these phenomena are precursors to more rapid bearing and gear tooth loaded surface failures.
I would give Redline's Shockproof Differential fluid a shot. At under $20 for the stuff and the 14mm Allen wrench you need (Sears) it may be sufficient to reduce ball skidding (thicker oil film) and metal to metal contact (thicker oil film), which reduces the source of the stimulation. The higher viscosity and additive package may also help reduce the resonant ringing you hear by increasing damping. But, any damage already done will not be undone, you will just slow down the rate of wear from where you are today and possibly reduce the audible symptoms you find objectionable. If the damage is enough to upset the preload on the bearing elements so much that skidding continues, well, the time you have bought is probably not going to be of real value.
I have used Redline gear lubes in my cars for over a decade and find they work very well at quieting gear noises, including differential whines. My 1986 190E 2.3-16 developed a nasty whine at 65 mph, a pretty common highway speed. It was about time to change the differential oil anyway so I tried the Redline Shockproof stuff and the whine went away. Not a scientific study on the subject, as another version of the product that was not "shockproof" might have done the trick too.
I can't tell what your actual situation is, but I would be inclined to change the fluid if it has not been changed in the last 30,000 miles. By the way, I am not affiliated in any way with Redline, I just like their products. The website if you are interested is:
Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)