for a mk2 on 17" wheels to lock up under hard braking and go sideways

i was on a greasy road, and braked as a car pulled out in front of me, the front locked , which i expected and controlled, then the rear locked , and the car went sideways, so theres me one handon wheel other on the window, applying opposite lock infront of the bemmer that pulled out in front of me, dont no who was more worried him or me.
cars lowered with konis, and mostly polybushed, running some nice new 17" alloys i got a few days ago

ive never had this happen to me before, but have a sneaking suspiction i should have a look at the rear bias valve on the axel.
any ideas welcome
oh yeah and can anyone hot a piccy of my car show i can stick it on here.
cheers jim