See this post:

http://www.peachparts.com/Wikka/W124Differential

I discuss removing the axle a bit here.

rorypff is correct - 10mm xzn, I purchased mine on eBay but most Autozone/Checker/Pep Boys have a set of these as well.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE