Seriously good advice has been given here to take the filler plug out first.
I didn't take that advice when draining the diff on my previous 87 W124 wagon.
Then I chewed up the harder-to-access filler plug.
After wailing for an hour or so and tearing out considerable amounts of hair I finally went to the local wreckers yard and found a spare DRAIN plug, drilled a hole in it to accept the plastic tube from the 1 litre bottles of oil, and squeezed the bottle to force about a pint of oil into the axle. Then a very quick swap-the -plugs-over dance, (yes I got very oily) and after a couple of abortive tries at it, managed to get enough oil in there to last me another 40,000 miles.
Yeah, undo the FILLER first. Just to be sure.
E200 Wagon 115k miles