Like Machz said, your shock is probably bottoming out. Check your travel and see if you need to extend your bump-stops or get different shocks.
Now we have bent the shock mounts when they are that low on a couple of vehicles. Everyone in the club runs the shock mounts off of the spring pads on top of the axle to keep them out of the way. You can install taller shock towers, allowing you to run the long shocks without introducing the bottom of them to the rocks.