FirstOnRaceDayAg said:
Check the axle pivot bushings (end of I-beam away from wheel/tire). Replaced the bushings on my 77 and tightened things up nicely. Good shocks will help as well. Sway bars are getting harder to find for these, but I did install front and rear on the 77 several years back.
Ditto on this. I drove an '82 F250 all through college. Replace the bushings where the radius arms and the I-Beams connect to the frame. Also check all the tie rod ends. This should go a long way to tightening things up. If you DIY, expect to work pretty hard. A come along will help to pull the radius at back into place. Otherwise, take it to an alignment shop.
If that doesn't fix it, you are probably looking at replacing kingpins and/or the steering box.