My 1982 Silverado half-ton two-wheel drive has a THM 350 automatic transmission leak that I can not figure out how to stop.  It leaks only when it is parked on an uphill grade with the engine NOT running. It is leaking from the center of the front yolk on the driveshaft, transmission fluid that is. It is not leaking from the rear seal as I replaced it when I replaced the worn out front u-joint. Does anybody know why it leaks from the center of the yolk and what I can do to stop the leak? Do I need to pull the tailshaft and look for a gasket or o-ring inside there or is there something missing from the yolk itself? All help would be greatly appreciated.  

YOU SCREWED UP THE SEAL WHEN YOU PUT IT IN--->WRONG--->YOU SHOULD BE ASHAMED THAT --->YOU WILL NEVER BE ABLE TO PROPERLY MAINTAIN YOUR OWN VEHICLES LIKE I DO<----  

If your transmission has a removable tail shaft,4 bolts screwed in from the back can be removed. Then you will separate tail shaft from trans housing and you can remove rubber o ring and replace with a new one.You should inspect both housings for cracks before installing parts. Also, you should replace speedometer o rings and seal that speedo gear shaft runs through.  

By the sound of it the bushing in the extention housing needs replacing. You'll need to get another seal too. Best advice is pull the extention housing bring it to a good tranny shop with the driveshaft so they can size the bushing properly and then your leak will be a memory also if the O ring seal that goes between the extention housing and rear flange on the tranny is in question replace it while your there.  

Hah!

You all aren't listening. The leak is coming right from the center of the casting in the yoke fwd of the u-joint. Through the casting itself, right out the center mark in the casting. Spray it clean, J-B epoxy it and forget about it. Just did one myself.