That hole is supposed to be there- - - -it's there to prevent an air lock developing as the slip yoke moves in and out as the suspension moves up and down. The fluid level should never get high enough in the case for fluid to leak out of the pinhole in the yoke. If it does, there's a faulty torque converter "anti-drain back valve" in the transmission valve body that allows the torque converter to empty itself after a few days of not being run. The fluid level rises high enough for a seep leak to happen as the fluid level gets above the centerline of the output shaft.

The quick fix is to braze up the vent hole in the plug at the end of the slip yoke. The right way is to fix the valve body leak.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!