AHHH Now you are talking John! I would guess, by your description, that your problem is internal in your differential. It's a good chance that you have a problem with a main differential bearing or an axel bearing. I had exactly what you are describing in one of my trucks... It was the main diff bearing. It allowed the diff to move a small amount and vibrate. It always sounded different when I shifted it into neutral.... The pressure is off allowing it to rattle at a different frequency.
