Are you running a loader with this tractor? The bearings shouldn't wear out so fast. Make sure that you lube the steering gear with corn head grease. It originally called for oil, but there was always a problem with leaking. The corn head grease won't get stiff in cold weather. Also, check to make sure you don't have too much play in the rest of the steering components. Seems like you are putting too much torque on those bearings somehow. 

email me if you need any info pstanley ( at) shaw (dot) ca