1989 Ford F-350 single rear wheel mileage: 30,000. I am experiencing a considerable sway getting up to highway speed and on deceleration but not so much at cruising speed. I have new ten ply radials, balanced, proper inflation, etc. I have three inch springs, six leafs. I have utility body, welder, torches, and tools on board, but am well under gross vehicle weight. The leaf springs do have a little squat to them. Would helper springs be in order or should I be looking elsewhere? Also has rear anti-sway bar.
Your swaying problem may be due to worn out or weak shock absorbers. You mentioned new ten ply radial tires we are assuming that all four tires are new ten ply radials. If only two tires are radial and the other two are bias ply, this most likely is the cause of your swaying problem.