Try adjusting the brake balance - also, you can apply a little throttle at the same as the brakes, this usually helps a wayward rear end.
You can try things with the setup (can't promise they will work 'though
). Less rear rebound damping. Consider a little more front spring or front compression damping.
On the diff - adding coast side diff lock can tame a wavy tail (on a trailing throttle or braking).