Here's some things I've run across on 2 different vehicle. On a 81' Datsun 4x4, I had a thump when turning at intersections at low speed. It turned out to be a bad lower ball joint. I don't think that is your problem since I think the Ford is a pin & sleeve style of "ball joint". On a Camero race car we had, when turning hard in one direction, the steering wheel would get real stiff (not good!). We took the top plate off the steering box and found one tooth broken off on the pittman arm. I Don't think this is your problem, but definitely worth a look. Before you remove the cover, I'd jack up the truck and put jack stands under the front axle as close to the wheels as possible. With the engine off and the key on, turn the wheels "lock to lock" and it should move very easily with no rough spots. Then with the help of someone (one person should be holding the steering wheel), check all steering components for play/looseness. You also might want to remove the brake calipers and check for play in the wheel hubs. What sounds like the problem is first in the steering box, when the steering doesn't correct itself after a turn. I would suspect too much preload on the gears inside the steering box. A quick way to check it is to loosen the lock nut that is located in the center of the top cover. Be sure to hold the set screw with a allen mrench when you loosen the nut. Then back out the set screw(remember the starting spot) till you have no tension. Then turn the set screw back in till you feel a slight pressure. Recheck several times till you're confident that you've removed any lash between the gears, with VERY little preload. Compare the original start spot and the final spot of the allen wrench and that should tell you where the preload was at. If that doesn't fix it, I would suspect that something is trying to "tell" the steering box to stay turning, probably something in the steering box. Not sure what to think if the power steering pump is providing too much pressure.

 

I'm going to third the steering box idea. If it's too tight it won't return on it's own.