If changing the rear axles is anything similar to a W123('81 240D),
it shouldn't take you more than 2.5hrs.
The most difficult part is jacking up the car and removing the wheel.
Put the car on jack-stands; drain the 3rd member; take off the wheel.
You'll find a single bolt in the center of the brake hub; remove it.
Pull the back cover on the differential; you'll find a "circ-clip"on the axle in question; slip a piece of thin wire through the hole in it( if you drop it, you can fish it out); use a needle nose and pull it out.
Grab both ends of the axle and push to the center.
The axle will fall to the ground.
Put a little anti-seize on the splines of the replacement; pick it up and expand it into position.
Put the single bolt back in the brake hub. and snap the circ-clip back into the groove of the axle.
Replace the rear cover(put a thin film of RTV on the mating surfaces) back on and re-fill the differential.
Put wheel back on and jack-down the car.
I have the "HAYNES" manual, and they give a blow-by-blow description.
Good luck. Regards, John F. ('81 240D & '85 500SEC)