Is this a power glide,350,400 or manual? The first thing I would do is block the front wheels and jack the back of the car up. This will help stop any fluid from coming out the back. Unbolt the rear straps and remove drive shaft. Theres a couple of ways of getting the seal out. Hammer and screwdriver or a tool used for pulling seals. I have never heard of the drive shaft getting out of balance. On newer cars when pulling 4 wheel drive shafts out I like to put them back in the same position. If this is a turbo 400 and the seal is leaking more than likely the bushing that the tail shaft rides on is wore out to. At this point you will need to pull the rear housing off and have a new bushing installed.