The key to taming the rear end is to keep the power on! If you need to slow down or the back starts to swap, don't let off! Keep the power on and learn to drag the rear brake.
Slowing the rebound kills your traction in the soft stuff. I suppose it solves the problem but only because you are going slower.
The faster a person gets the less rebound dampening they need. The only problem with this concept is that when you make a mistake and chop the throttle :)
Mike, work on learning to drag the rear brake with the throttle on. It's the holy grail for going fast in the sandy whoops and braking bumps.