My 2003 Chevy Blazer will not shift in to 4 wheel drive HI or LO. There are 3 buttons on the dash to choose from, so it's all electrical. It was working within the last few months. The other day I needed it, and nothing. Couldn't even get up my own driveway with a little snow on it. I'm getting the same clicking sound I got before, when it was working, but it isn't going in to 4 wheel drive. I miss my old manual Tracker and GMC pickup with lock-ins and a 4 wheel drive shift lever.
Any thoughts on where to begin are appreciated.
Thanks.

 

My $.02 worth is shift motor.

 

I have had problems with moisture in the vacuum line to the actuator that stops 4WD from engaging. More of an issue in cold weather when the moisture freezes but still a potential issue anytime.