Check to see if there is a groove on the output shaft where an o-ring should be. Many times that o-ring gets forgotten or left off (or dislodged upon installation of the yolk). Make sure you know if the output shaft uses a "O" ring or not by removing the extension housing. If it has the O ring land, then find a yoke that was made for the O ring. The yoke will not have splines on the very end so it will slide over the O ring. Yokes that are designed to work with the O ring usually have a hole in the plug in the end. The O ring seals the transmission fluid from entering the splines, so you have to grease the splines, and the hole vents the air that is trapped in the yoke. It's pretty common to not have the O ring in place and get a nice big leak out that hole.