Rusted froze driveshaft

Ohter than that.. pb blaster, and go out and tap it with a hammer on all sides about every 15 minutes.. rotate and tap. Don't bang hard enough to damage your bearing/ seal in the gear box.. just enough to loosen rust.. etc

If it comes down to a point where it just won't come off.. and the gear box/mower is good.. take a hack saw or a die grinder/angle grinder, and cut the yoke off. Cut into it but stop before you hit the mowers shaft... do it on opposite sides.. or even 4 sides.. should make it pop right off.. or at least open it enough for you to cold chisslle it off without harming the mower bearing/seal/shaft. Then you'll need to get a ney yoke end and slap on the u-joint.

If the shaft is galled or pitted.. dress it very well with emory paper, or a file for large raised galls. get it buttery smooth with fine emory paper/cloth before you slide another yoke on there.. lube it to. Try to just dress the surface.. you don't really want to remove material to significantly lower the OD of the shaft, as the shaft to yoke may wobble.

Soundguy