check for inside snap rings. keep soaking them. sometimes part of the snap ring will come out, and some breaks off and is still holding the cup.
a ball joint press doubles as a u-joint press, but if they are that rusted, it will bend the two sides towards each other...unless you have a piece of metal of the right size to separate the two cup holes from bending towards each other. even then it's risky. a good vice and the right size sockets will do the same thing.

my favorite way is the "smart hammer" method. it's never failed me yet. i even repaired my bent yoke with that method once. (of course, i am the one who bent the yoke, using the press method)
if they are really stuck, the "smart hammer" can dimple the shaft, but to get it balanced costs about $60 and is nice to have done anyway.