Tom:
I am not a tech, just an avid DIY'er but I have replaced differentials in both a 124 and 201 body.
The bolts that hold the axle to the output flanges have 12 pt stars so you'll need the correct size for those. There are 8 bolts per axle and you'll need to be able to turn the axle as you go around to remove the bolts. This means that you need some good jack stands to hold the car up while you're doing this...or if you have access to a lift, that would be best.
There are only three bolts that hold the differential to the subframe. Two at the back which use the 12mm allen heads that Benzmac referred to and one in front.
You'll also need to remove the ABS sensor. Oh, I think it's much easier to do all of this if you remove the exhaust from the cat back.
Good luck
