This comes up from time to time. A long time ago, I found a link to a seal that some vehicles use.
Mostly Jeeps, I think.
Its a piece of rubber that has "teeth" on the outside to match the splines of the yoke.
I'm guessing that it sits against the end of the output shaft and acts as a seal to prevent the fluid from getting much into the hollow yoke.
Why some vehicles would need such a thing and others not is a good question. I suspect that most would not need one until the splines got worn enough for the fluid leakage to be noticeable.
Where you would find one I have no idea. I imagine you could make one out of a medium hard rubber casting compound.
Put lots of release agent in the yoke, with something to block it from going too deep, then pour in about 1/2 inch of the stuff and let it harden.
Trim up the result, slide it in and out of the yoke a few times to break it in, then stick it to the end of the output shaft with some light glue or heavy grease.
Speaking of which, putting a good amount of really sticky grease in the yoke might help things out for a while.
Just enough to fill the gaps in the splines, not enough to actually fill the yoke. Air still has to get in and out as it moves.
