96 Jimmy no 4 wheel drive

Of course I have to learn that my 4 wheel drive (high or low) doesn't work in the middle of a snowstorm. I have the floor shift lever with the button. I can shift into 4h and the little indicator light goes on, but the drive train diagram next to it stays in 2wd where it used to make the 4wd diagram light up. I also tested it in front of some store windows to see that only the back wheels spin when I goose it, fronts do not. I've read some advise on vac issues, but unfortunately I can't work on it in the middle of a storm. Would this generate a code if I use my reader, or any other tips to look for?

Thanks in advance,
Jeff

also, is there a common cause for the back left stereo speaker to not work before I go digging it out?

No codes for this, as you say check for issues of vacuum or a bad actuator on the ft axle.

No codes for this, as you say check for issues of vacuum or a bad actuator on the ft axle.
thanks....will check it out