It was -35 F when my wife turned the key and the ignition actuator part broke. After searching this site and a couple of nights after work, I finally got it replaced. Thought I would summarize what I had to do because I never did find the complete procedure in one post.
1991 F150 Supercab. Auto tranny - tilt steering - cruise.
After it broke the symptoms were a freewheeling ignition lock mechanism. It was stuck in the run position although the truck didn't start. The running lights stayed on so my wife disconnected the neg battery cable. I could reach down and work the actual ignition switch by manually moving the rod which runs between the actuator and the switch along the top of the column tube so theoretically I could have started it ( but the battery was run down and it was still really cold). I had to remove the two screws under the steering wheel cover and then unsnap it and disconnect the three wires underneath and remove it. I then used a 15/16 socket to remove the nut on the wheel and a steering wheel puller to remove it. I then removed the two screws from the signal light assembly and one screw from the wire close to the key mechanism (buzzer?). I reached down and fed the wire loom up the column until I had enough to clear the shaft with the signal assembly. I used a small nail to push the lock cylinder retainer which was on the outside of the column just above the hazard switch (it may be inside the column on a non-tilt) and pulled the key cylinder out. At this point from what I've read on this site you can replace the actuator now if you have a non-tilt column. Mine has tilt so I found a couple of small fine threaded bolts to fit into the pivot pins on either side of the tilt mechanism. I screwed them in a far as they would go without force and then measured a small socket and washer so that it was slightly longer than the protruding bolts. I then unscrewed the bolt and ran it through the washer and socket and tightened it up which pulled the pin out some. I had to add another washer to finish the job. After removing three screws and the snap ring on the shaft (Use snap ring pliers or make sure you have a new snap ring) I could then pull the tilt part of the column up aways. My acutuator arm was broke in three pieces the long shaft was still in the square hole which I worked out with a mini screw driver. The half moon part was attached to the actuator rod and I had to use a small blunt ended nail to drive out a little roll pin to take the rod off. I backed up the mechanism with a screw driver berfore tapping it out. The other piece was attached to the slot on the end of the flat gear that is driven by the brass key cylinder gear. This fell off when I pulled the flat gear out. I then cleaned thing up in there a little sprayed a tiny amount of WD40 on the moving parts and put the new actuator arm in. I had to pull back on the tilt assembly to get it to go in the square hole. ( I also tapped the roll pin partway in before I installed the actuator). The first time I installed the tilted mechanism I couldn't get the lock cylinder to go back in because the slot in the brass gear wasn't aligned with the slot in the washer. I had to pull it back a couple of times and turn the brass gear a little then try again until the two slots lined up.
After that I put every thing back on in reverse order and it went fairly smooth.
Sorry for the long post but thought it might help someone to find most of the procedure in one place.

 

Thanks for the info.