904 or rear axle runs jerky help!

hello i have some problems with my dodge b250 van-88.
my problem seems to lie in the automatic gearbox or rear axle
when you shift into gear and drive slowly it starts to jerk really bad, the engine runs good, no problems there. where could the problem be? the oil in the transmission is old and a bit dark colored.

I know its not an a-body but thought you might be able to help me anyway