97-00 doesnt care about the rear o2 sensor for fueling. 01+ have the cat diag in the PCM that when the cat and rear o2 are removed will effect gas mileage.
On 01+ the cat can always be removed, but will require a tuner to shut off the diag and the code, the 97-00 will just need a tune to shut off the code and stop the SES from coming on.