Sounds similar to the exhaust for the newer Mustangs. I think they make a 'MIL eliminator' that essentially plugs into the harness for the O2 sensor and fools it into thinking that its still there. Look at some of the mod motor vendors (Steeda, Saleen, Paul's High Performance, etc.) and see if they think it'd work with the Ranger. If not, you could consider welding an O2 bung to your off-road pipe for the sensor and see if its happy even without the converter.
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Jeff Chambers
Trophy Stock #3
11.611 Seconds / 116.7 MPH
Chambers Racing Team