I don't know, but it sounds similar to a problem I had with an 87 Dodge Ramcharger when the front axle wasn't completely disengaging after taking it out of 4WD. Only it wasn't just a steering wheel vibration. It vibrated the whole truck.
A lot depends on your build date, but Ford has a TSB out for a problem with the early discs and calipers. TSB 05-21-19. This MAY be the problem in your case. If still under warranty, it would be replaced for free.
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