If the transmission has never been touched, the outer torque ball retainer is metal which formed sort of seal with the torque ball. Replacements are better with rubber vulcanized to the outer torque ball retainer.
External leaks are messy and can be monitored, but internal are a bigger problem: the seal at the end of the torque tube was originally leather and is past it's prime. Leaks here allow fluid into the torque tube and sometime into the rear end by way of a defective pinion seal (leather also).
The available outer torque ball retainer kits have the torque tube seal also. I drill a hole in the torque tube about 1.5 inches in front of the rear flange to monitor fluid in the torque tube.
You can slow down minor leaks by adding a (blue) bottle of STP.
The factory service manual is your friend!