parts questions

Hi guys, i am not too knowledgeble with parts and their duties. Can you explain whats the purpose of ball joints and my mechanic said that my catalyc converter may need to be replaced. is there a way to fix the catalyc or do something with it in order not to replace it. kind of costly.
Also, what is the shock in the back of the car, on the right side for, its kind of at an angle, right under the exhaust pipe.
i got a 1998 explorer xlt, v6 4.0 automatic
Thanks for any help

Ball joints serve as the pivot points between the tires and suspension. Ball joints also support weight. On some vehicles, the ball joints may be used to make wheel alignment settings.
You can not fix a catiletyc converter you must change it. Call around for qoutes.
I have been told the shock that is there for an anti roll stabilizer.