Most of the time -1.5 degrees is said to be optimal for both 2wd and 4x4. But I think less, maybe -1 degree, might be better for a 2wd. IIRC the manual "How to prepare the Sierra for motorsport' mentioned the settings they used on the 3dr for different rallies. I think in the end they started using less camber on the rear as it probably helps traction in a straight line.