P0420 Error Code on Jeep GC Laredo 4X4

Help. Had the catalytic converter replaced last dec. Under recall. Had the muffler replaced this summer. Check engine light came on last week. Went back to the dealer and they said it was the downstream o2 sensor. Took it home, bought one and replaced it. 2 days later same thing. The dealer says I got a bad one and that it cant be the converter. The engine seems to be awfully hot after short drives. How can I tell if the converter is bad without going back to them? The jeep had 187,000 when they recalled the converter and I never had any trouble with it. Now, I have 205,000 and nothing but issues. Its a 4x4 and most of it has been replaced. Just did the ignition coil and the ecm was replaced by the dealer in june of '05. I also noticed last nite after it came on (when I parked it and shut it off) that there was a lot of "crackling" noises coming from underneath the jeep towards the center. Any ideas?