I would check the mass air sensor to start with. I had the opposite problem, mine likes to get a little lean and doesn't like to run on ethanol mixed gas at all. When it intermitently gimps out I get no power over 3500 and I get an airfuel lean code. I tested the mass air sensor and it is bad. In my case I have attributed the sensor failure to an oiled filter I have been using that fouled up the filiments on the sensor...
I don't think your 2WD ECU would cause any problems.
I'm sure some of the other guys will chime in here too.
