So I've got an ignition issue of some sort. When you turn it to "on" the lights come on and everything. Turn further to "start" and the truck starts just fine, but when you let go of the key, instead of the key kicking back to the on position and the truck keep running, it kicks all the way back to "off" and shuts off the truck.

If you hold the key at the "on" position after the engine has started, the truck stays running fine...

Which part of the ignition is it that needs replacing to get it to stay running again?

 

I would bet that the actual, real ignition 'switch' mounted atop the steering column is failing. It has a spring-loaded wiper that makes/breaks connections as it slides along. The 'start' position is opposed by the spring, so that the switch will return to ON when the spring is allowed to move the wiper.
You can take the column cover off and see the switch further down the column from the lock cylinder, operated by a rod on the tail end of the lock cylinder. Mine was literally coming apart due to the twist of the wire loom putting pressure on it from day one. They are inexpensive.
Removing the 'security' bolts was not difficult using a file and a small pair of vice-grip pliers. I replaced with normal bolts.
tom

 

Check with Ford to make sure your Ranger is not included in the ignition recall. However, I'm pretty sure your vehicle is not one of the problem childs.
I used to own an '88 Cougar that had the same symptoms as what you described. It needed an ignition switch replacement and Ford issued a recall on it. I remember driving it to the dealer holding the ignition almost to the "start" position so it would stay running.

 

I actually started looking into it yesterday by pulling the lock cylinder and I think you're right. What I can't find however is what that part is called, or a part number. Among the forums is mention of lower and upper "actuators", but NAPA, Advance Auto, Autozone, and O'Reilley all can't find the part by that name, or even just searching for ignition parts.

I'd have called the dealer but their parts departments are closed on weekends.

I also can't order on-line for lack of a part number.

As of yesterday I can't even hold the key in that position to keep it running. The switch in there has definitely gotten worse..

Anyone have part numbers for this, by chance?

 

You're making this too hard.
Go to RockAuto.com.
Now, click on "Ford".
Then click to your year, I guess you said 1993.
Click on "Ranger".
Click on your engine size of "4.0".
Then go to "Electrical-Switch & Relay".
Click on "Ignition Starter Switch".
Now you can look at all the selections they have, looks like about six different numbers. I'm not advocating ordering from RockAuto here, it's just that you can print some photos or part numbers to help you when you do order your parts.

And I really am against you ordering parts before you have it all apart. What if your problem isn't the ignition starter switch?

 

Well, upon inspecting it today, with proper lighting, I can see that the spot the end of the lock tumbler goes into is worn. So maybe that's not the only issue, but it is AN issue and will require replacement anyway.

I can't simply leave the truck where it is because it's street parked, and on the wrong side of the street for this coming Tuesday's street cleaning. With part number(s) in hand I should have an easier time finding the part locally, hopefully today.

I've also got my Volvo on blocks doing the timing belt, so I've been a bit scatter-brained the past couple days. Started the Volvo to find out the next morning that the truck won't stay running, and am trying to get everything sorted.

Thank you so much for the link!