i no this is a commodore forum but my mates falcon is shuddering constantly it does it when he brakes which could be the disc warped it does it under acceleration too which i suspect is a missfire but not sure and it does it when decelerating with out braking
Could be a knackered uni joint!
Or a wheel out of balance, though that would probably be more at high speed.
I'd be checking the uni's first, grab the tailshaft and see if there's any play in it.