[thenorm]

have you had your car aligned recently?
many times that wiggle in the back can be caused by the toe being off spec.
as you may know, the focus rear suspension has that "control blade" which dynamicly changes the toe-in under suspension compression, which is why, when u hit a bump, the toe changes momentarily and the car scoots sideways a bit. this effect is often more noticable in inclement weather. This is alwso why you must get an alignment after putting on lowering springs.

the spec for toe in the rear is from 1/8" toe in to 0 toe in. (or u can be like me and put 1/16" toe out for extra rotation during auto-x). one advantage to setting the toe to 0 is that if you load extra wieght in the car, the toe may still stay in spec, but if ur at 1/8" the extra weight will put the toe past it (due to the control blade effect)