This is an output shaft speed sensor. My check engine light came on a couple weeks ago, so I took it to an auto parts store to get a free scan (who knew this existed?), and the result was p0720 Output Shaft Speed Sensor Circuit. So, after hours of research and many visits to jeepforum.com, I decided I could fix this myself. I took three trips to Napa Auto parts this morning, and $42.95 later, I finally had my hands on the correct part. This lovely output speed sensor. I shimmied under my car, which was an incredibly claustrophobic experience because you're actually supposed to raise your car to work underneath it (who has time/knowledge/tools for that?), located the part on the transmission, and, after a small battle with the electrical plug and a wrench, I removed the faulty part and installed the new one.
And now, instead of a check engine light and a little jolt when my car tries to shift, my car won't even start. Did I mention that owning a car is the WORST?