Further, 19.5"s as someone else said DO tend to wander some at first, depending on brand. Do you have at least 1-2K miles on them at this point? Which tire brand/model are they?
Both new sets of 19.5"ers I've bought wandered at first. The Michelins calmed down after 1-2K. The current Hankooks wander more than the Michelins did, but not very much.
Didn't the earlier F550's have a narrower front end than the later ones?
The first thing I would do is go weigh as a truck stop. If your front end is lighter loaded than unloaded, it will cause the truck to steer poorly and wander. The hard data will help answer that. $8 and you will know.
Is your truck 4WD?