There are a number of things you can do for more rear end grip. The easiest is to drop your rear ride height a mm or so. This will also give you more rear camber. You could install some adjustable rear links so you have a way to increase rear camber.

The problem might not be at the rear of the car, but at the front. It could be the front is biting too hard and digging in the outside front tire. Try raising the front ride height a mm or so to keep the weight transfer down. A rear roll bar is helpful also cause it helps control the roll of the front. You could also try reducing the amount of travel and/or slow down the servo speed. You can also widen the track by using spacers.

Rear end chatter is usually one or a combination of the following: A mismatch in the shock oil to spring. Too stiff a spring or too light an oil can do this. Not enough rear camber and ride height in the rear being too high. Rear track too narrow. Too much rebound in the shocks.

Probably more things, but those are the most common errors.