Thanks, Jkanly. I guess I should have been more specific. When I took this 231 apart there was a rubber washer of sorts under the nut that holds the front output yoke on. I was unable to find this seal washer, tries RTV, and now the splines are leaking a lot. What I am trying to figure out is how to seal those splines? Thanks.
RTV will work if you clean off the splines on the shaft and the yoke with brake cleaner, then let it dry and put on the RTV. Let the RTV set up for 5-10 minutes before you assemble. FWIW I've never seen that crazy washer on any other yoke. By the way I use Dynatex RTV , I found it much better than the permatex autozone stuff.
To answer my own question, and to help others who might be interested, the way you can seal the fron yoke on a NP231 is to use grey RTV. I used black when I assembled the case and when I took it apart it was still not set up. I guess the transmission fluid reacts with the black for some reason. I sealed it with 3M grey RTV making sure that there was plenty of it on the splines of the yoke and under the nut, immediately put fluid in and have no leak at all as of now, 2 days later.