I recently had that happen on a 88 F350, the first time it was track bar bushings, replaced them and it was good.
Then the next time it did it I replaced a couple tie rod ends and a drag link, it didn't fix the problem. I then realized it may be the tires, I took them in and one of them was separating, new front tires and problem solved.
I had the same issue on a 85 chevy k30...new tires fixed it.
