Harsh ride!

[NoHachi]

The easy thing to try is to set the rear ride height back to where both 4 links are pointing slightly upward again (looking in the direction to the front of the car). Watch out though, you'll notice some extra oversteer.
If this works you either have too little travel or the traction brackets might be worth a shot.

You want to confirm that you are not hitting the internal or external bumpstops. Can you take a look and see how much bump travel is left with the car in static position (how much exposed shaft). You need a good 30mm or so at a minimum. Are the rear dampers shock body adjustable? Do you see any evidence of spring bind on the springs (small nicks and scratches).

Checking for binding like totta mentions is also a very good idea. Its hard to do in practice though, since you cannot replicate the forces on the bushings easily. However you should be able to freely move one end of the axle or the other through its range of motion at a minimum.