Thanks-- Sounds like I've got me a weekend project!!! Hope it works---

 

I should have pointed out that the ignition switch is not the same thing as the lock cylinder. The lock cylinder is simply the receptacle for the key, and contains little more than the tumbler assembly. The tail of the lock cylinder engages the upper portion of the ignition switch, a multi-stage switch that connects via a cable to the lower half of the switch that contains the connectors pins for the wiring harness.
This isn't intended to discourage you, just to clarify what it is you may be replacing.
Also, if you have access to a dependable VOM with a 2-3 amp rating, you can do a quick check by removing the ground cable on the battery and holding one lead to the battery terminal and the other to ground cable. (I mention the amp rating because many inexpensive VOMs are only rated at 200-300 milliamps and will have their internal fuses blown by attempting to measure larger currents.)
Have a friend, spouse, passerby, temp hire, mailman, whatever, cycle the ignition switch on and off a few times. With the key on, you should see a spike in current flow as the electrical system handles all the various things that go on when you first turn the ignition on - not the least of which would be running the fuel pump for 2 seconds. It should then drop, but not to zero. (I don't know what it would be after everything settles down and the pump stops running.) With the ignition switch off, it should be a very low draw, as you still have things such as the clock and a few other things running. If, during some of the times when the switch is off, you see a significant amount of current being drawn, then that's about as good a guarantee as you're going to get that the switch has failed.
Oh, and for the love of God, remember that you are working around the driver airbag in this case. Be sure to read the necessary steps to disarm and remove it before getting started if you do change the switch.

 

Guess I should update on this project...

It was (1) a bad fuel pump and (2) a bad alternator... who'd have thunk?

The tank was a pain to get out and in, but with enough pushing and wiggling and beer, eventually got it. Alternator... free replacement under warranty, and a whole 20 minutes later, all done! Or so I thought...

The starter squealed. Took it out, tested it, turns out the bearings went bad. Well, another free replacement and afternoon and beer later, now it's good to go!

These trucks are fun!

 

Jeff - would the ignition switch problem you're describing also cause my Jimmy to periodically stall in traffic?