You can either source an O-Ring as stated above or drain the T Case, pull the nut and washer, apply some RTV in the base of the pocket and reinstall the washer and nut, let it dry for a few hours and install new oil. When I rebuild them we do the RTV for added measure and have had no leaks there. We have found that the yokes tend together a grove from the seal and when I rebuild them now we just replace the yokes at that time with a new seal etc. We also have had leaks on the bolts that hold it together and we use a non hardening sealer like Permatex on all of the threads to take care of that as well.