My TJ station wagon has a ground lead dangling on the rear underneath of my chasis at the rear (approx where the differential would be) what purpose does this lead have, why is it there ????

Assuming it isn't an actual cable that has come off something.. they're generally used to earth the car to the ground to prevent ground leak buildups and whatnot. They also stop you from getting zapped when you step out of the car :)

There is an earth strap under the car that bolts on at both ends this helps reduce corrosion through electrolysis. I can't remember exactly where it bolts to anymore.