Was in the seat of a 4020 in 1st going through a groundhog village with a 15' field cultivator. Came out the far side of the village, clutched & shifted to 3rd, gently started back up & it took off like normal but then stopped so fast the fan squealed as it spun in the stalled belts. I slid back up into the seat & it was exactly as you describe. Started and ran but would not move no matter what! Had to pick it up & carry it to the shop. It was the top shaft forward gear synchronizer as others have said. There was enough wear in there that it went into both the high and low forward speeds at the same time. We opened him up, got it unstuck, split the tractor, said goodbye to many pictures of Mr. Franklin as we replaced many parts in there and put it all together.
Gained even more respect for the machine! Any machine that can lock up so suddenly yet not actually break anything deserves respect! So too those who built it!
Had forgotten how nicely they shift when not worn out! That was like 10 years ago & it's been sweet ever since.