Have a friend help you.
Crawl under the front-end, and with the engine running (for PS), have your friend rock the steering back and forth while you look at everything. Start with the steering box, and look carefully at the so-called "frame" to see if there are any cracks...the whole steering box could be moving. If this is it, check the web for reinforcing plates and install them - welding may not be required.
Next, find the track-bar. It runs from the axle to the frame. Watch both ends, look for failures of the frame mount, and especially look for failures of the rubber bushings inside the ends of the track bar. The only way to get bushings is to replace the track bar.
Those are the two most likely causes. The other causes should also be visible using the same quick test, but also check with the wheel locked at each end. Rock the steering into and out of lock and watch everything. Something is moving.
Good luck,
Crawldaddy