My book shows W123 starting in 1977 but it clearly shows a step called "Replacement of radial sealing ring on drive pinion" - #35-1550. It looks like you should be able to replace it without going all the way through the differential. Does show a pretty elaborate looking puller to remove the drive pinion but doesn't look like anything someone couldn't put together from bolts/puller flange, etc.. You can probably do the job a little more economically. My problem with this type of job is that when I take this stuff apart I want to fix everything
that is worn out.