Button behind the igniton lock.
 

This is probably a noob question but I don't have an owners manual for my truck but I hope someone can help..
Theres a hold down switch behind where I put the key in, without the engine running it's possible to hold it down about 2 inches from where it sits. When the engine is running though, it seems to be locked off and won't travel more than about half an inch.
Does anyone know what thats for, I'm sure most of you do actually.
I can take a pic if my explanation wasn't clear enough.

Thanks in Advance.
Jamie

What are you working on here? Do you have a picture of the area in question?

Ah sorry yeah, I forgot to mention its on my '81 GMC Jimmy.
I've figured it out though.
You hold it down when you want the ignition to lock off.
I read somewhere, I think it was in the Haynes manual, that you shouldn't be able to remove the key without the ignition at the lock position but I can take out my key at the point before that and also at the point before it cranks :P I'll try and take the key out with the engine running later and I'll report back haha.

*Combining Consecutive Posts*

Ok, Time for that update.
It's possible to remove my key at any turn of it, even with the engine running, I can still take out the key haha. I also found that so long as the ignition isn't in the lock position, I can start the truck without even needing the key. It seems for my truck that I only need the key to get the ingition cylinder out of the locked position, after that I'm good to go with out the key in there at all :P haha.
It's probably not a good thing to be able to do but I think its pretty cool haha.

Sounds like your ignition lock cylinder and/or key has worn out.

I know that there are some of those vintage trucks, both C/K and S/T that had a lock on the key that prohibited the key from being removed without first pushing down on a lever. That is likely what you are describing.

That lever was only on manual transmission trucks.On automatic transmission vehicles,its built into the steering column shifter.

My '80 stepside, manual trans, had the same button/lever and like yours as long you didn't lock the ignition, or doors, no need for keys. lol.