99 SS question
3 channel cars have a toner ring inside the diff which has a sensor mounted on top of the diff housing,
4 channed cars have 2 abs sensors at each end of the axles with toner rings pressed onto the axles,
to convert a 3 ch rear to a 4 ch is easy, you just find a cover for the sensor hole, then gring out a v shap at the back flange at the axle housing ends then just put 4 ch axles in there., your done,
to convert 4 ch to 3 ch takes a lot more work but can be done.
check the stickys in the gears and axles forum

