I just got a 1986 Trans Am with a 700r4 auto trans and when i put it into gear it lunges hard. My friend looked underneath and saw the driveshaft turn about 1/3 turn in the rear diff , is this a problem with the rear ,or trans any ideas ,im on dirt and it tries to spin the tire for a slit sec when i engage the gear, i havent gone down the road with it yet but wondered what would cause this thanks

does your car idle high or is it normal idle even a few hundred rpm's could cause a harsh slam into gear in both forward and reverse you will be able to tell more after a short road test as far as rear end problem you can jack the rear of the car up put it in nuetral and use your hand to turn driveshaft and check the lash(free movement back and forth) it should only turn a little in each direction before you feel resistance