True Pete, it still being driven but the force is reversed. It's almost like the diff. is trying to push the tranny. Could probably be a bad bearing or maby diff. internals. Just my opinion.
Wehn power is applied, something might be lifting out of place inside the diff. and (maby a gear is loosing its position ? very slightly though.) And when power is taken off, it goes back into place. I know I have the weirdest comments ! Don't even ask me why !