Yoke seized on 400 tranny.

[57Suburb]

Bought a used 400 tranny a few weeks back. Had a yoke on the tail shaft when I went to pick it up, and I assumed it was there to keep fluid from running out. Didn't bother to pull it out at the time because I didn't want to make a mess. Nothing to worry about, right? Well, bolted it up to the motor I'm dropping in my 72 and starting putting it in the old truck last night. I reached the point where it was almost home, and it was time to pull the yoke and slip the one already in the truck on the shaft. Gave it a tug and nothing. Grabbed a rubber mallet and gave it a couple swift blows, nothing. Grabbed the small sledge and gave it a few more "taps". Still nothing. Wow, never knew it was possible that these yokes can seize up on the shafts. Is it really seized? Tranny is pretty clean externally, nothing to indicated it has sat outside where moisture would be a problem. Hate to beat on it very hard for fear of damaging something inside the tranny. Anyone else ever had this problem? Any tips for getting this thing off? Grrrrr!