There is a ton of posts on this subject here and elsewhere. If there is no flange connection near the diff, you have homokinetic. But you can also use annular.

Do a search here and on Google.

For a start: http://www.peachparts.com/shopforum/diesel-discussion/220000-w123-how-replacing-rear-axles.html

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5