Axle bearings front and rear on 4000hr 9200s turns into $25,000 repair bills for each end. Injector cups and dropped valves on engines not as common ,The axle  bearings  seem to be the weakest link any grease or oil leaks on the inner wheel  and you can figure out why they are for sale. I think the 60s and 70 series still had the grease type outer axle bearing like the older tractors. The newer ones have oil  type bearings and the problems are worse.