Yes the slip joint is sometimes a square shaft. There may be decades of old hardened grease and or rust preventing the slip joint to move. Try to clean up that section and apply a liberal amount of penetrant. The woodruff key on the shaft at the yoke may also binding. Gentle tapping on the yoke end at the PTO is fine. Try using a drift so that the hits are clean and precise, tapping in a direction away from the PTO. Tap one side, rotate the driveshaft 180 degrees and tap that side and repeat as necessary. Use care as you don't want to break any difficult to source parts. No need to loose patience and go Neanderthal. Good luck!
