01 QX4 Catalyst Advice?

[Badmofotx]

I have a 2001 QX4 with an issue. I have seen a few posts on the codes P0420 and P0430 which is what mine keeps giving. I have had exhaust shops say that I need both cats replaced just from what the codes say. I prefer to do that myself but the dealer wants like $800 each...not looking to do that! Some say that the post cat O2's need to be replaced first...I have crawled under it many times and have not located any post cat O2's to replace. I guess I want advice on whether to butcher my exhaust with a local shop replacing the cats, or try to locate aftermarket bolt ons, or replace the sensors. What do you think? P.S. it has over 100K and not covered under warranty.