Final (hopefully) update.

I took the driveshaft out (even though there was essentially no play in it), and lo and behold the double cardan joint was (completely) effed - see pics. I must have been moments away from catastrophic failure...

After removing the driveshaft and locking the center dif, no further noises were heard! Ordered a new driveshaft, and I guess I've got a RWD disco until it arrives!

 

Good news. Thanks for keeping us updated.