Do a search on 1/2 shafts and learn all about it. I am not sure about the 4-matic but the rear differential should be just like all the rest.

a. no
b. 6 - straight forward but a little physically demanding.
c. I can do them on the 123/126 in about 45min/side.
d. Make sure the car is in the air securely and take your time.

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Jim