If your scanner can read live datastream look at what the O2 sensors are doing, if the up-stream O2 is fluctuateing real good then it is working and you need a cat, if it is not fluctuateing very much or at all then replace it before doing the cat.

A dirty upstream sensor can overwhelm the cat and make it apear bad.

A trick I learned recently was that if a guy cuts out the cat and replaces it with a pipe and runs a new upstream O2 and an old down stream O2 they will rear different even though there is no cat and keep the light off, At least on a 97 ram 2500 with a 360