Check rear suspension bushes, exhaust mounts + bolt tightness in general.. worst case scenario - excessive backlash (space between gear teeth before they make contact) between the crown wheel and the pinion inside your diff, can be adjusted with careful precision if you can safely raise the rear of the vehicle and drive it, a mate can use a mechanics stethoscope (or a metal bar/screwdriver ) and place it on the diff housing (don't inhale exhaust fumes or get caught in spinning wheels!) to listen for excessive grinding, knocking etc (very thorough way to check how your diff is going)
