Have a JD 1250 farm tractor that's 29 years old. Both gear shift levers can shift into gear with no problem when tractor is off. When tractor is on, the rear lifting hitch works fine which tells me the internal gears are working fine. But both gear shift levers can't shift into gear when tractor is on. When I try to shift it into gear all I hear is the internal gears grinding. Is this an indication of an internal clutch problem or is there and external adjustment I can do to shift it into gear. Never had this problem before and no prior signs of clutch problems. Its my work horse and I need it back up and running. Any help would be much appreciate.