The security light doesn't come on except during the normal start-up. I had something happen a few years ago when the starter wouldn't crank at all using the key. Having grown up on tempermental starter soleniods I did the old reach through the dust shield with a screw driver and jump the battery lead to the solenoid terminal trick and it started. I'm beginning to circle around to a bad switch. How hard is it to replace??

 

The ignition switch is relatively easy.

Here is the book method: Ignition Switch Replacement - Book Method...

From what I remember of the few replacements I have done, you basically unbolt the kick panel under the dash & remove it. Remove the upper and lower shroud off of the steering column. Disconnect the main wiring harness. Remove it from the clip and then remove the individual connectors from the main center connector. You will then remove the other electrical connectors on the old ignition switch and remove it from the column.

 

I had this exact same confounding issue with my 2001 Blazer. Check the fuse marked TBC Truck Boby Computer. Its located in the engine compartment fuse box right hand side. Fixed my problem immediately. It seemed incongruous but this blown TBC fuse effected other componants as well like the interior lights, engine compartment light, driver's side door lock. The engine would fire on one or two cylenders and then imediately shut down. I had the steering column shrouds all off ready to do battle with the ignition switch when I traced the fault to this fuse. Good Luck