Originally Posted by Retro Grouch

Does that derailleur have a hanger bolt that involves a spring and a snap ring? Is that what you're talking about?

If that's the case, I've replaced a few Shimano equivlents in my time. It seems like it would be easier to do if you had three hands but there isn't room for even two sets of fingers. I'm not aware of a special tool. It just takes patience, a steady hand, and the use of some words that only bicycle mechanics are allowed to say.

Good luck.