It`s been quite a while for me also replacing an axle. Probably close to 6 yrs.
They are a tight fit. Don`t remember which end I replaced first, but I replaced the "C" Clip last.
I also had the Differential unbolted from the body on a floor jack to raise or lower. There is the 4 13mm bolts that holds the Diff Mount to the body, these are what I remove.
If you remove the 2 in the center of the mount, they are 22mm as I remember.
Here is a DIY link to replacing the Differential by DELIVERYVALVE several yrs ago. Shows how he R&R`d the axles with pictures.
PeachPartsWiki: Replacing the Differential
Is yours a one piece or two piece axle?
If you have the two piece, (Annular style) you can remove the 6 bolts, place the Stub in the Diff. and put the Splines in the Hub, then rebolt the two halves together.
A few more links
http://www.peachparts.com/shopforum/diesel-discussion/220000-w123-how-replacing-rear-axles.html
here is the Wiki from that thread.
PeachPartsWiki: Replacing the Rear Axles
Here is a Google search I did with more info
https://www.google.com/search?q=site+peachparts,+replacing+w123/w126+axles&biw=942&bih=449&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiOgbba7cnPAhUSzWMKHYG2AM4Q_AUIBygC&dpr=1.45
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