Siezed Rear U joint

While replacing my flex disk and CSB on my rear drive shaft on my xi a while back, I noticed that the rear most U joint (closest to the rear diff) was seized up. At the time I was trying to eliminate the typical CSB shudder under hard accel. so I put the drive shaft back up as is, taking out one problem at a time.

Now I have a definite vibration coming from the rear and when I let off the gas and coast at higher speeds, it hums back there.

Does this U joint sound like the culprit?

Siezed Rear U joint