I haven't heard of an active differential, but what about some brake based torque vectoring? Anyone know? That would be cheap and easy to implement, and would be an awesome match to the active suspension.
In my experience brake based torque vectoring works well until you start to move through the normal wear and tear of the brake pads. Then, depending on how close to the replacement level you are, the car starts to experience some odd behavior as the pads just can't apply the same level of "force" they once did.