1989 Ford F-350 sways a lot while accelerating and deceleration

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.