I finally got a chance to really look at it, and it turned out to be the driveshaft (prop shaft). It was odd though, because, when the car was sitting, it would somehow cause the center bearing to lock so it didn't feel loose at all. I only figured it out by getting under it, with the left rear on jack stands, and having someone spin the left rear wheel while I watched the driveshaft rotate. It got to a specific spot, then it came loose. Got another shaft at a u-pull yard for $23 on sale, and the vibration is gone.