It's the sensor.
Put in the AEM and it reads normally. Low 10's (there's some room for some tuning here, hello!) under load and normally around town. When the factory sensor goes to 20+ on the highway the AEM is relatively unaffected -- goes richer as a matter of fact as I guess the ecu attempts to compensate for the bad readings.
Bad news is even though this is a factory sensor I bought last week, and is "under warranty" -- it just got so butchered when I took it out (bung on the crappy ebay headers stripped it) that I doubt they're going to honor the warranty when I show up with it with the threads all chewed up and the faces of the thing eaten up by my vice grips.
Son of a. Looks like i'll be welding the bung that came with my AEM somewhere and plugging that son of a bitch up while I'm at it.
...unless, of course, it's the ECU -- but I hope not. Looks like the end of the sensor got bent a bit by the crossmember when the motor torqued due to the aftermarket placement of the sensor... That probably caused the problem, if I had to guess.