do you know which cat would tend to go bad 1st ? I assume the pre cat would.
When mine went on my old 97 it was the main cat.
If you pull the cre cat 02 sensor out and it gets better then thats the side you have a problem with. If it gets better regardless of what side you pull then start looking at the main cat.
Or you can just do what I did on my grand cherokee. Buy a infrared temp gun and check the temps. My good cat was showing in the 300 range while the bad one was pulling 500+, the core was completely fricken melted when we took it off. I replaced them both anyhow with a set of hi-flows. I went from 17mpg on the highway to 21-22, they wont take long to pay themselves off!