Have them do a quick check by lifting the front end so both tires are off the ground. Turn the wheels to one side (almost to the stop), lock one hub and make sure the other is unlocked. Now rotate the tire of the side you locked. If the joint is siezed the tire will not rotate and will try to turn the steering. Do that to both sides (remembering to lock and unlock the hubs). If everything checks out, pull both front hubs and test drive, try to recreate the situation when the problem occured. I have seen the factory Ford hubs not release into 2wd. To remove the hubs, first spray around the hubs with a light penatrating oil, pinch the retaining ring and wiggle/ pull the hub. It maybee stuck in place (the rubber O-ring is stuck), in that case you can use a rubber mallet to knock the hub around to work the oil in. You do not need to remove the tire to do this.
Also per Ford, have them push/ pull on the tie-rods (checking the ball and socket joint) and lift up the boots on the tie-rods and inspect for rust. Rust means replace.
Have the ball joints been replaced recently? Is the steering damper bent or dented (would cause the shock piston inside to get stuck).