On the shaft in the trans inside the tail housing there is an oring that seals to the inside of that yoke.
See how shiney the yoke is on the inside before the splines start?
That's where the yoke slides on the oring that seals the fluid in the trans.
They used those yokes on GM trucks to relieve the pressure build of from the yoke having to move in and out on bumps.
We just plugged the hole and the outside edges of the plug with epoxy when we ran across those on cars.