Help! Starter disabled on truck due to alarm after battery change

Try pressing the brake pedal while turning the key. I think that's the kill switch for those. If not, I'll try harder to remember.

 

I know for a fact it's something ridiculously simple like that, I just can't remember the trick specifically. I've encountered that exact thing on several late nineties s10's before and when I figure it out I'm always like "Damn, why didn't I try that." At least 10 times...but I still can't remember.

Also, there's a possibility that the new battery for some reason blew the ignition fuse. I can't tell you exactly where that's located, but look under the hood in the relay box...there's a ~6"X8" black plastic cover on (I believe) the driver's side of the hood area. That just pops off and reveals your relays. Read the labels in there, I think that's where the ignition fuse would be.

But odds are it's just that factory ignition kill. You can also try taking the key out and putting it in and on to the position right before start. Leave it there for 10 seconds or so and try starting combinations like that.

 

Also if your key is a replacement and doesn't have the little black square on the shaft of the key like this:

then you could've reset Chevy's PASS-key thing and it's looking for the correct chip (the little black square) and not finding it.

EDIT: I think you can reprogram that PASS-key system to match your current key if you have the little chip on it by leaving it in the on position for like 10 minutes. Not sure about that though.