I'll take a guess...
When was the last time you checked the alignment?
V Drives don't allow for much variance. If the shaft has been bent due to poor alignment it may be possible it finally gave way. I read tons of info this year about shaft alignment and drive-train components when I was dealing with similar issues. As strange and odd as it sound, I read about 1"+ shafts breaking from poor alignment. I did read about the crevice corrosion that SD speaks of but everything I read directed the cause being shaft seals like the PSS (I believe the reason they now have the vent tube installed?), you stated you have a traditional stuffing box. (not disputed you SD, just say'in)
Also, when was the last time you replaced the engine mounts?
Did you notice excessive vibration or noises from the transmission/engine?
Do you have any oil leaks in and around the rear seal?