I have had some tires that made a truck pull from one side to another on rough patchy roads, but most times there's a issue with the steering.
How much free travel is in the steering wheel, it should have less than a quarter turn, 1/8 turn is better but it does need some free travel. Check the free travel in the shaft next to the steering box as there's a couple of ujoints in the steering shaft between the steering box and steering wheel that can give a false reading.
If the steering wheel has to much free travel check these.
tie rod ends
Steering box adjustment
I've worked on, owned and driven several Ford L series trucks and have replaced more worn drag links than any other part on the steering.
ujoints in steering shaft
slop in telescoping shaft
If steering is tight check these
king pins, jack up the front axle then pry up on the tire to check for movement
alignment, should be toed in 1/8 inch
wheel bearing tightness
spring eye bushings and pins
spring shackles and pins