The end of the yoke is sealed with a flat metal welch plate...they leak sometimes...
Usually no need to replace the yoke....unless the oil seal area is grooved up beyond repair.
Remove the yoke from the driveshaft by removing the caps in the yoke that center the front universal joint.
Clean the yoke up, then knock the metal flat plate using a small shaft driven from the spline end of the yoke...keep the flat plate to reinstall...
Clean the internal area where the plate seats and clean the plate itself...I use brake clean...look close at the flat plate, it will have a dimpled area on the outer side from where it was installed originally,,,,use a ball head hammer to knock the dimple back out. This will shrink the plate overall.
Mix up some 2 part epoxy and coat the sealing area of the yoke where the plate sits, then reinstall the plate with the convex side out, then expand the plate into the seal area by hitting the center of the plate with 1/2" drift. This will seat the plate back n place and the epoxy will seal it all up.
Spread some more epoxy around the outside of the sealing area....
Let the epoxy dry totally...
Then reassemble the driveshaft universal joint...
While the shaft is out, replace the seal in the extension housing...
