Yes putting a 2wd ecu on a 4wd car will do this, the 2wd ecu can't run with a cat, as it has no control over it, it is also getting the wrong info from the throttle position sensor, the 4wd ones work the opposite way to 2wd, ie one goes from high to low and the other from low to high when opening the throttle.
It sounds a right bodge tbh.
tabetha