No special tool that I am aware of, certainly didn't need one when I did them on my cossies.

You first need to remove the split pin (bit of wire running through a hole in the thread).
Then, a 17mm socket should get the nut off, I would try and get some WD40 or similar on there first though.

Make sure you get new split pins to put in there when you fit everything back together, they're there for a reason

Oh, you might need a balljoint separator once you have the nut off though, to separate the two "bits". Be careful not to damage the rubber seal on the bottom "thingy".

Sorry about "bits" and "thingy", had a long day and I can't remember what they are called