first make sure your yoke is long enough to engage the oring seal inside of the tailshaft housing.. sometimes driveshshafts get replaced and are not the right length .. inspect your yoke shaft and make sure its not deeply grooved. the yoke seal is easy to replace, just remove the 4 bolts on the tailshaft housing, pull it off and you should see a metal sleeve, with an oring in it,, many times, if the transmission has been rebuilt, the metal sleeve and washer will be missing,, I think people hammer on them to get them off, and destroy them,, they are thin metal. then they just dont bother to find another one. it would be very difficult to weld the vent hole on the yoke with the ujoint in place.. Furthermore,, most GM ujoints are not held in place by C clips, they have hot glue melted into a small hole in the u joint, and you have to destroy the ujoint to get them out,, you have to melt the glue out with a torch,, its quite the shitshow... then you have to try and find the correct u joint to replace it..