It sounds as if you have missed one or more of the lubrication points along the way. The lubrication needs of ride-on mowers' steering systems are extensive, and are not covered by grease nipples: most of the crucial points need to be can-oiled. In general I would suggest you start with the steering column bearings, then the king-pins, then the steering crank if it has that type of steering. However if your king-pins are the grease nipple type and you have kept them greased, the most likely spots seem to be the steering crank (which I think is can-oiled, though I think Greenfield should have considered it a good place for a grease nipple) and the steering column bearings. The tie rod ends are the least likely culprits, but need to be checked of course.
