So I went to pull the driveshaft out to repair the leak coming from inside the slip yoke. I notice the nuts at the yoke on the differential weren't especially tight. When I removed the U-bolts the caps came off the u-joint quite easily, as the needle bearings had been pounded into paste. The joint was well lubed, but the bearings were shot. The ones on the driveshaft side were just fine, no damage. Also, when I attempted to push the d/s forward to clear the rear yoke, it would only move about 1/2" and stop. After some screwing around with it, I got it out of the rear yoke and pulled it out of the transmission.
 Upon inspection, I found that the  internal splines of the T5 slip yoke I got off eBay had twisted a few thousandths just beyond where the output shaft ends. That's why it wouldn't move forward. Nice soft Chinese steel......... but it was a good price...... I now have a new Spicer slip yoke on order, along with a new Spicer u-joint.
Can't figure on the bearing failures - I never had any vibration issues at all with the driveshaft. Always really smooth. Go figger........