rear-end problems???

ok, i have a 96 police vic (cvpi) and i was on the expressway the other day going about 75-80 when all of a sudden the car actged like it was trying to stop on its own. it was like jerking and stuff. so i parked at the nearest exit's gas station and it was still doing it, even if i left the car in nuetral and let it roll. it was even making a grinding noise from the back of the car around the rear universal joint and rear end yoke. so i had a couple of people look at it and they "think" it is the rear end. has anyone else had a problem like this. i am going to buy another rear end cause i dont know what else to do. i am three hours away from home and need to get back asap.