Axle shaft should not be too hard. I haven't done the job myself yet, but am about to replace the shafts on my W123 in the next few weeks, and I've found several well detailed how-to's. It looks fairly straight forward.

Here is the How-To on PeachParts:
W123 A How to, replacing rear axles.

Here is another one from a blog with good pictures as well:
http://mercb-w123.blogspot.com/2013/06/canging-rear-axle-driveshaft.html


Intermittent banging though:
Are you sure it's not your drive shaft? Check the flex discs and the carrier bearing...

Also, a word of caution: I don't know if it's a valid claim, but I've read online that folks say to not mix&match between the homokinetic and annular style of rear axles. Apparently having one of each makes the differential unbalanced and can break it. Again, I don't know if the claim is valid, but it sounds plausible to me, and I thought I'd mention it. Do with it what you want. Personally, I'm planning on swapping out both rear axles for the annular ones.

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)