You have center one-way? Make sure its not on the wrong way or your car will be just rear wheel drive.
Make sure your tires are in good condition. Have you tried using softer in the rear? Your front belts, are too loose? loose belts result in teeth skipping that can off balance the power distribution.
And like everyone else said, check for any tweaks in the chassis. Take your shocks off, make sure there are no bind in the suspension travel(check for bent shock shaft as well). Take your pinion off and spin your drive train. you should be able to spin all four tires just by spinning one.