You have the diff set to rear? This could be part of your problem! For normal driving, you should have the split set to about 40/60 or 45/55 (Front/Rear). If you have it set to the rear, you're effectively driving a rear wheel drive (2WD) car, in which case lifting mid corner will result in more oversteer (back end sliding out) than with a AWD car. Try setting to a central setting to see if this helps!
Anti-Roll Bars (ARB) are a metal bar that links the bottom of the suspension (just by the wheel) from left to right - there's one linking the front and one linking the rear. The idea is that some of the forces from the suspension on one side of the car is transferred to the suspension on the other side. The stiffer the ARB, then the more force is transferred and the car will roll less due to the difference in suspension compression on either side. If the ARBs are not balanced (same front and rear) then you will either get oversteer if the rear is stiffer than the front or understeer if the front is stiffer than the rear (if they are both the same, then you will normally get understeer due to the extra weight at the front of the car). Factory fitted ARB's are not usually adjustable, however most replacement rear ARBs are adjustable - if you have an adjustable rear ARB, try setting to a softer position (closest to the outside of the bar).
Another thing to check is the drop links. These are small metal or plastic links about 3 to 5 inches (depending on the model) which connect the suspension strutt to the ARB. The plastic ones in particular are prone to snapping, especially if the ARBs have been uprated. If a drop link has broken, then that could seriously upset the handling.