George is correct, have seen this before. Oil is migrating down the splines and exiting the back plug in the yoke. Since there is no oil pressure there, you can repair by cleaning the area and apply silicone around the edge of the plug.
Seems to me I heard a few years back, someone coated the spline grooves in the slip yoke with grease to create a dam to help keep the trans fluid from getting to that yoke plug. ^^^^^
That plug area sounds about right as far as the leak.
