One way to at least see what is going on is to open the small cover over the pinion just in front of the left brake. It should be held on with two bolts. This should allow you to see the pinion and whether it is latched in or not. Should be able to spray the pinion with penetrating oil etc. I have heard of tractors that had no cover there, but that would have been earlier models than the U series I believe.
If you can't accomplish anything through there, you might be able to do something through the main clutch adjustment/inspection cover, but it will be a long way over to the side.
Pulling the pinion and clutch assembly is the final answer! If you are unable to latch the pinion in, you may have to remove more hardware up on the governor and injection pump housing than normal to get the pinion to come all the way out. The radiator has to be drained and the starting motor unbolted and lifted up a few inches to allow the pinion to clear the idler gear hanging down from the bottom of the pony motor.
Fairly common problem....remove cover as CC states and get in there with a big screw driver or pry bar between the engagement lever and the end of the engagement sleeve. They usually submit with reasonable force.
CCJersey and OM, Thanks for the tip. Is the cover the two bolt cover between the clutch and the pinion housing, on top side? Thanks, Jim.