Most of the time oil leaks past the welch pressed into the slip yoke. If you have replaced the double lipped oil seal and it's still leaking I'd be sealing up the uni joint end of the slip yoke with some epoxy.
I had massive trouble with one years ago and it turned out to be this.
You can also remove the spring from the oil seal and cut a little off to increase the clamping pressure around the slip yoke if it has worn a groove.

I just luv my "clacker Jabber".................