I feel a little uncomfortable about putting a torch on the differential fill plug, but I can't explain why.
Maybe you can have some success with cleaning around the fill plug and then using penetrating oil. To get mine out the first time (on a '82 300D with plenty of room to work) I had to use a pretty good sized 1/2-in drive ratchet and my feet while holding on to the rear bumper.
I think you just need a little more leverage on the wrench.
(They really don't need to be all THAT tight.....