Can I suggest the centre propshaft bearing?
I had a noise that I thought was the diff going, but it didn't change with different loads as such.
If I remember correctly it was a 'hissing/whine'.
Ended up being the centre bearing, changed that. Sorted. Easy and cheap fix too, compared to the diff.
Thanks, good suggestion. It had occured to me too, mentioned in my second post. My only experience of central bearing trouble is the rubber mounting failing and the bearing banging about. But I appreciate it is a ball bearing that can fail, and become noisy. The noise I am hearing is new to me, and I have had diffs fail before. Also the location is not so clearly rear of car, the wife placed it more central, where the prop shaft bearing is.
Hissing whine is how she described it.
Another reason to perform the Entwood location test