Wish I would have seen your thread earlier....P0420 almost always means that you need the Cat replaced. I send everyone to the local muffler shop, they can usually replace any truck cat for around $150 total. They will usually take the cat out, look inside, call me and say "That cat aint plugged up!"....and I'll say, "I know, just replace it, please".
Anyway, P0420 means that the cat isn't doing what it's supposed to do and the O2 sensor right behind it knows that it's flowing too freely and sends the message to the PCM and sets the code.
Personally, I think it's a stupid code and the OEM's (and possibly the govt.) should really loosen up the criteria for the PCM setting the code.

Travis