Don't mean to hijack your thread mate, but my car does the same sort of thing. I'm under the impression i have a locker diff, it's just very inconsistent on when it wants to make some noise. It gets people looking at my car for the wrong reasons.
But what R33 racer has said has me thinking now, as "sometimes" I can feel the shudder actually beneath me from the rear of the car, other times it's just at the back. I have had the oil changed to redline lightweight, but i need to change it again to a heavier oil. Lightweight makes it good for less friction, but i don't need it for racing!