2000 Volvo S40 catalytic converter help

I'm hoping someone can help me with this. The second catalytic converter (closer to the rear of the car) got detached from the exhaust pipe of my wife's Volvo S40 today. I think it's something I can handle, I've just never worked on one before. The dealer quoted $750, but Autozone has one for $260. They said that I would need the whole piece, that I wouldn't be able to just re-attach it. Is that true? Any help would be appreciated. Thanks!