I have a 91 chevy with a 4l60 trans. The slip yoke has a hole in the middle and a dust plug /seal that is leaking trans fluid.I could try and replace the seal but i'd rather get a newer yoke that is solid all the way thru.I was told buy a guy at the local junkyard that the hole was for a mechanical speedo and that electronic speedo equiped trucks used the same shaft they just plugged the hole. thanks AJ.

The hole in your yoke is there to prevent an airlock on the geartrain which can put enough forward thrust to destroy your planetary gears. If you remove the extension housing you will see a cup that is pressed on the output shaft. This cup has an o ring in the inner portion to seal the driveshaft yoke and still allow air to escape when the driveshaft is thrust forward protecting geartrain. Simply replace the o ring to stop your leak. Do not use a sealed drivesfaft yoke.
mike

Thanks for the answer Mike.If I understand what you said the leak isn't at the plug but inside the exstention housing and it's just leaking out the dust plug.I didn't think about an inner seal because the outside of the slip shaft
is clean with no sighns of trans fluid.My speed sensor is leaking as well so should I replace the uv joint aslong as i'm under there? Are the slip yokes on newer trucks made the same way? thanks again.

The seal you are refuring to is the on on the left on the output shaft right?

Great picture AJwaverider.
The o ring is housed directly where it points to output shaft. Ujoint does not cause that leak. Old rubber does. I am doing one of these repairs in the am. Very somple job.
mike

Thanks AJ.

Mike L . I finally started taking apart the tailshaft and replaced that oring seal.It took 10 minutes or so to get the old seal out and 5 seconds to put the new seal in .The old seal was hard and stiff like plastic .I had to use a set of dental pics to get it out and that was the hardest part of the job.I replaced the speedsensor seal but had to buy a new speed sensor because the old one didn't survive removal and the catches for the connecter was broken anyway.I knew the uv joint didn't cause the leak but I asked if I should replace it because it has been covered with trans fluid and unlike the rear uv joint can not be lubed.Like you said,simple job .I should have it back together by the end of the week and will give you a update on it's conditions.
thanks again.

Glad I could help.
mike

Put everything back together yesterday and took it out for a test drive today.No leaks and the trans is shifting smooth,also managed to fill the tranny to the proper level the first time.:)

Another job well done. Transmission repair is not rocket science except the stuff we can't figure out; sometimes that takes an act of God.