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!