I had a similar problem with my Dakota once. There was a wire broke off the O2 sensor. It was dumping LARGE amounts of fuel in and trapping it in the Cat. After replacing the O2 sensor and putting a Magna Flow high flow Cat on it all was well again.
if you try driving your car at night, set up some ramps so you can get a good luck under it when you return. Drive your car right up onto the ramps (power permitting) and you will see it glowing if thats the issue. you can't miss it...