The problem you are having is due to the spool in the back. You will be far better off with a open diff or a limited slip carrier out of a Toyota.
The massive under steer you are getting is due to the rear axle always wanting to push the car in a strait line. When you make tight turns in autocross, one tire has to spin faster that the other on the rear axle. A spool wont allow this. This is fine in dirt where you can get lots of tire spin to compensate.
Also, in tight turns on pavement, a spool with load the suspension by twisting the axle housing into the radius arms, then to the chassis creating goby of body roll and unpredictable handling.
Best bet, buy a new 3rd member with low gears and stuff a posi or limited slip * same thing* carrier in there and bring it with you just for autocross.
Max out your negative camber for autocross, the more the better for super quick cornering. Adjust the caster as needed depending on what tires you want to use. You always want a good return to center feel when you come out of the corner, but not so much that you can unsettle the suspension.
