That's not much to go on but I can list the common causes. On any Ford product start with a steering and suspension system inspection. Ford has a lot of trouble with ball joints and tie rod ends separating leading to loss of control and crashes. If your truck still has their infamous "rubber-bonded socket' design tie rod ends, get rid of those before one falls apart. If they're loose already, that will allow the wheels to oscillate back and forth. Hitting a bump in the road will set up the vibration, and the loose steering parts will seriously aggravate that.
If there is a steering damper on your truck, check for oil leaking from it. If you see that, replace the damper. If there isn't one, you can add one if necessary.
The twin I-beam suspension is very strong but it is by far the worst design for tire wear. To keep that wear to a minimum, ride height must be maintained as specified. As the coil springs sag from age, the tops of the front tires will tip in on top. That makes each tire want to roll toward the center of the truck. When one tire hits a bump, it momentarily has more weight on it, and that makes the entire steering system want to follow that tire. That bump lasts just long enough to bounce that tire and start the oscillation. Any tire and alignment shop will have a small book that shows where to take the measurements and what they should be.
The last condition is excessive "caster". That is one of the three primary alignment angles. If you stand to the side of the truck and look at the wheel, just inside of it are the upper and lower ball joints. The upper ball joint is a little to the rear compared to the lower ball joint. That is called "positive caster", and has been used since the 1960s to increase road feel and reduce steering wander. The drawback is it makes the truck harder to steer, so we added power steering to make up for that. Positive caster is also what makes the steering system return to centered when you let go of the steering wheel after making a turn. The higher the caster is adjusted to, the faster the steering system returns to centered. Steering returnability is a good thing but you can go too far. When the system returns too quickly, it overshoots and goes the other way, similar to a clock pendulum.
Caster is very hard to adjust on your truck, but it usually doesn't need to be readjusted except maybe once in the life of the truck. If this problem started right after an alignment was done, double-check the "before" and "after" caster readings on the alignment printout. If you post them here I can interpret them.