I no longer own a W126 body machine, so I cannot go out and check it out. But, I doubt the differential housing design changed much, as the 14mm Allen hex configuration has been used on MB drivelines on 220D and 240D cars that I have worked on since 1971, and the 1986 190E. I am presuming you are talking about the differential plugs.
The fluid change is necessary on a regular basis to remove wear and chemical breakdown contamination of the lubricant that will build up in the differential. This is a unit that is cooled and lubricated by the fluid, as it is splashed and sprayed all over the inside of the housing by the driveline turning. There are no filters or other means to get rid of wear particles, and when it is hot and the lubricant is loaded at the gear tooth interface by shear and pressure forces, the oil molecules break up and become something other than oil, with no where to go. I believe regular changes at 30,000 miles is recommended in your manual. I go for 60,000 miles using RedLine products. When I change it, the oil is darker, but not ugly. No problems to date. That whine started after hittng 200,000 miles, and it went away after changing the fluid.
Hope this helps, 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)