I'm 99% sure it is differential oil. What complicates matters is that I had a transmission line blow in May and it threw tranny oil everywhere on everything from front to back. However, right above the rear u-joint there is a wide spray of thin oil. You can see the difference between this and the grease from the U-joint and other stains. I can also smell that distinct gear oil smell.

Looks like this requires some tools to remove the seal and bearing. I'm worried about that bearing because I've had a constant nagging vibration problem and It seems to be coming from the driveshaft or differential.