Driveline vibration out of now where! Help!

Look through the hole with a bright flash light to see if you can see rust or bearing damage.

Put your finger up in there on the tail shaft and slowly turn the driveshaft with your other hand in the proper forward rotation. If there is a bad bearing there you will feel the roughness in it as it conducts the vibrations to the tail shaft.

Prossess of elimination:

If you can determine that the tail shaft bearing is still good by this method but still feel the clunking on the tail shaft then you know the damage if farther up the line into the transmission. And now it's time to pull the pan and check it out.

Any suspect transmission problems I have on any automatic car or truck, the first thing I do is drop the pan. This tells you the whole health of the transmission.