Bad O2 sensors are common on 98 and 99 J30A1's. I would replace the O2 sensor and then see if it still gives you the cat error; it could have tripped the cat code as well. The O2 sensor is kind of pricey, but we can be reasonably confident that it's gone.

I'm not sure if your header is to blame or not. My guess would be no, but to find out you'd have to measure the A/F ratio.

As far as the cat, mine has been throwing the code intermittently for the past 30k miles. I passed emissions last fall so I just ignore it when the CEL comes on and I see that I have that code :dunno: