The selector shaft seal CAN be changed with the trans in the car. I think KD made the tool I have for removal and installation.Where is the leak? You got a cable operated trans. There is a kickdown shaft on top, about 2 inches from that cable entry, pointing straight up. It takes a lipped seal, and they do go bad all the time. Bad thing is you need to pull the pan/valve body or trans to pop it out. Its what that lokar cable is attached to feeding from the rear.
There is a shift cable O-ring on the cable adapter, I bet its bad. Remove cable bolt and pull out (watch fluid) I believe the O-ring is around the cable sheath end, youll see it.