This probably isn't the proper way of doing it, but I also had trouble getting U
joint apart on a NON LBC.
What I did was open up the vise far enough so it would support just the arms of
one yoke horizontally. I then used a hammer and a hard wood block with a hole
in it on the upright yoke and drove that yoke downward which forced the cup up
out of the yoke. You could probably use a large socket, but I didn't want to
take the chance of peening some metal over. Hammering on the end grain of a
wood block gives a more solid surface than the side grain. Of course there is
the change of damaging the needles and spyder of the U joint, but if the joint
is bad, then that is no worry. That is what I encountered.