The reason why your single key works in the drivers door and ignition is because they were the two spots where it was used the most.
Consequently, they have corresponding amounts of wear.
When i got my cruiser, I got one key, and through a misadventure, managed to lose the key within moments.
Out of desperation, I took a shot and discovered that my single Mitsubishi Montero key successfully operated both the driver door lock and ignition. That's how loose the tumblers in these locks get after almost three decades. Hell, even my plastic AAA wallet key for the Montero managed to work in the old Cruiser lock and ignition.
It is a snap to pop off the passenger door panel and pop the lock cylinder out. Frankly, it is WELL worth it to KNOW the lock number for your truck.
Even with all new keys, I found the rear hatch lock was occasionally difficult. I pulled that and took it to my locksmith and, being an old-school guy, he took the cylinder apart and "detailed" it. Cleaned, and each tumbler slide lightly gone over with emery cloth and replaced. Works like a champ now. You could do this yourself, if you manage to not lose any of the tiny springs.
While getting the code and makign new keys is one solution, note that the drivers lock and ignition are STILL going to be ridiculously loose. After an incident in which I locked my keys in the car, I discovered that the drivers door will open to almost any key that will fit - seriously, a wide range of keys will work.
So, if you are serious about securing your vehicle, getting NEW lock cylinders may be a much better bet.