Ok... Back to the drawing board. The new ecu didn't fix it. I have yet to double check that it's putting the correct voltage out to the coolant temp sensor. If it's correct with this new ecu, awesome, I'll keep it, and consider that another "bug" worked out.
I think I have to revisit the O2 sensor... I'm leaning towards that now, because I've noticed that my truck will start and run fine for a minute, and then start surging something fierce (and stalling). From my understanding, these engines use a heated O2 sensor, so maybe that minute of smooth running is just the amount of time it takes to warm up the O2 sensor, and when the computer decides it's ready for sensor input, it gets a stupidly lean reading...
I guess I can think about it till I'm blue in the face... Time to get under the hood... again.