I replaced my rear axles, on a '81 240D(w123).
It took longer to jack the car and drain the differential, than it did to replace the axles.
There is a circlip in the differential and a single bolt through the brake hub. Just collapse the axle to the center and it falls to the ground.
Put anti-seize on the splines lightly, and expand them into position.
Simple as that.
Do not get under the car without jack stands!!
Also HAYNES, sells a DIY manual for the w123's.
Good luck, John F. '85 SEC; '81 240D