I had the same problem for several years on 61 series 4000 with independent front suspension it had been to truck shops and tire shops for about 3 years and no one could find a problem, I had checked wheel run out tire out of round tie rod ends and toe in finally with it jacked up and prying just the right direction we found the lower ball joints were worn out, and it was hard to get it in the spot to find the wear for some reason, new joionts and it hasn't bothered me since.
