Slip yoke will not slip on...

I know you looked inside the slip yoke and it looks good - any noticeable knicks on the outside of the yoke that might hang up on the bushing?

Too much lube/grease and you're hydra-locking it on?
"Feel" each spline inside the yoke with a long pick of some sort to make sure they are all clear to the back?
The transmission is more suspect than the slip yoke i have used before... Just a light spray lubrication not a hydrological situation.. Hydro lock feels more like spongy that it won't go on further, This is sticking or jamming on something and having to take a hammer and beat it back off..