2005 Ford F-250 Wandering

Hi. My question is, I was wondering why my Effie wanders, at times fairly badly. When it first happened. I got it wheel aligned and checked by Trueline, who said it was slightly out and they replaced a bush of some kind which really made no difference. The last set of tyres (offroad wore very well and and lasted around 80000kms.). I now have a set of 22 inch road tyres which I hoped would rectify the wander but alas no difference probably worse if anything. Where would you recommend be the next place to check. Tahks in advance.
Bunster