I have skimmed through the thread briefly but just a quick shot in the dark...

Your driveshaft may be slightly bent/dented and/or your tailshaft bushing worn...

If your driveshaft is bent your slip yoke could be riding on the tailshaft bushing and it finally wore bad enough to cause the severe vibrations...

You can take a look at your slip yoke and see if there is any burn/wear marks on it..or if you notice it is smooth as glass and uneven wear

I would also inspect the tailshaft bushing very closely make sure its not worn or damaged.