Please help, Code 72

I keep getting code 72 - catalytic converter circuitry. It's a 96xj, 159,000 miles, 4.0l. A couple weeks ago, after the cel had been on for a while, I pulled, cleaned, and reinstalled the O2 ground connections on the block, under spark plug #5. It didn't clear the problem...the light came back on after resetting/clearing the codes. It appears to run okay, and mileage does not appear to be suffering any more than it has in the past. 17mpg highway with 31's. Has anyone else gotten code #72??? what was the fix?

Yes, it's a year later and I'm still getting code #72 on my 96xj. Has anyone else had this before? I'm now at 175,000 miles. To be honest, my 'aft' catalytic O2 sensor is original. I thought if it were toast it would throw a different code.