chevy venture catalytic converter\O2 sensor

I have a 1998 Chevy Venture van. The check engine light has been on for about two months. I took it to Auto Zone to get an engine diagnostic reading and the result was a code PO420. Seems to be either the O2 sensors
(it has two) or the catalytic converter. I took the one sensor out and checked it with a multimeter and propane torch, it seems to be good. I can't get to the other sensor to check it. I connected a vacuum meter and got a 19 at idle and 21 at around 1800 rpm. The van runs sluggish, but it will run 80 miles per hour. So, my questions:
A: Is it likely to be one of the oxygen sensors?
B: Or a "poisoned" converter. (no additives have ever been used in the fuel)
C: Stopped up converter.
D: If it is the converter, what is a rough estimate to replace it in the southeast. (Tennessee)
Thanks, Terry
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Have it analyzed with an OBDII tester. The O2 sensors can be checked. Running long periods with a bad O2 sensor or rich condition will damage the converter....

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