How close is the O2 sensor on the 4 cyl trucks to your former catalytic converter location? On my V6, it's very close.

Since the cats are noted for the heat they generate, could be the sensor is operating much colder than intended and sending a strange signal to the ECM.

When operating w/o the sensor, it was maybe using a default value that went back to the 'strange' value with the new sensor - IE, the original sensor is/was working, just sending a screwy signal (as far as the ECM is concerned).

Just a thought...I'll be interested in seeing what you finally find out, mine has been doing the same thing since I got it, but I haven't had the time or resources to do the level of investigation you have going, I've just been living with it!

Jay