Did you see if the u-joints were turning in the steering joints, have the planetary covers looked like they have been off lately on the front wheels. It could be a stripped input shaft yoke, stripped pinion splines, ring and pinion, rt/lt axle shaft from the diff, sun gear in planetary stripped or removed. I would look close at the pinion input splines and drive shaft yoke, then spin the drive shaft again and see if the u-joints are turning, leave it in 4x4 and raise the two front tires off the ground and try to spin the tires by hand and watch the u-joints in the wheel. Some of those parts are pretty high. Other things to think about, does it have a full cab, extenda-hoe, 4x1 bucket, some of these options are a trade off when looking at a repair cost and getting a deal with a machine. If your not hearing any noise you could pull the drive shaft and sun gears and run it like it is also and fix issues with it later. Did you check the drain plug in the axle and planetary lube levels, see any metal on the mag, oil look ok.