Does Park hold on a hill ? Have seen park pawl or tooth on park gear break off and fall down into mfwd gears and lock up front wheels. If you engage front wheels and drive, do you hear and tapping or knocking sounds in drivetrain? If so, I would drain transmission oil and remove the mfwd drop box from under the tractor. You can inspect gears and can look up into the transmission from under there and see the park pawl to inspect it. If broken, tractor needs to be split to replace it.