It could be the diff (but you'd expect that to still be complaining), but I've come accross cases of this on Disco's where it was the wheel bearing expiring, and it seems to be not uncommon. First time on a Defender though, why Disco's should do it and not Defender I don't know! The other equally possible thing is that the CV joint has exploded, jack the wheel up and see if turning it tries to drive the propshaft. Thinking about it, I'd vote for the CV joint.........
Richard