As above its very very unlikely to be the diff as the ATB ensures that the power is distributed equaliy, this would again lead me to look at the cars geometry.

In a straight line it should just leave big black lines if you accelerating from a stand still, but pulling you forward at the same time, rather than torque steering, and pulling to the left or the right.

From your description it sounds like the effect you describe is happening as weight is transfered to the rear(under acceleration) or as the rear becomes light (under brakeing). Which would lead me to believe its related to the setup at the rear being to aggressive(biased towars cornering). Hopefully your next setup will cure your problems...best of luck.