[JCoop]

Come to think of it, Bob, you're right. I guess I should have driven it after one change. Now, I'll never know what it was...

It DID also make but a slightly different grinding/groaning noise in 4th at low speeds, but only towards the end there. Like last week when I drove it like this.
1500 RPM and below. I know, I shouldn't really be lugging this engine but one of the redeeming qualities of the big block was being able to do it: loping around in 4th or 5th. It certainly sounded like it was coming from the back.

Would harmonics have anything to do with it, perhaps? Just that right combination of engine rpm/diff rpm? IOW, running 1500 rpm in 3rd turns the diff a diff amount than 1500 rpm in 5th?