Hi Could be stuck between 2 gears I never fixed this tractor it was a 4020, Guy just used me to find what was wrong for free, buy telling me the job was mine. Then fixed it with his buddy cause they couldn't figure it out!. He changed gear in the field to go into top gear I believe.
It went between the 2 ranges and stuck. it stopped moving so fast it put the steering wheel in his chest. The problem with his was the syncro piled and stuck the 2 gears together on the top shaft. I managed to lever and rock it back and forth to free it, with the trans top off. That was a high hour tractor but had low flow in the transmission pump and lack of lubrication didn't help it, from what I was told buy another guy later, that new him.