Blazer will not go into 4 wheel drive?

Answer 1

What year is your blazer? Most common cause for it to not go into 4x4 is the vacuum actuator. The actuator is run by vacuum from the transfer case. The vacuum compresses the rubber diaphragm which pulls a cable on the front differential to lock it in. The rubber diaphragm is prone to break down and if there are holes or tears in the rubber it will only suck air and not have the amount of vacuum needed to pull the actuator cable.

This actuator is commonly mounted under the battery tray. Easy to remove and inspect.

1998 blazer

There is a vacuum switch on the transfer case that usually goes bad. I have replaced mine 3 times. Factory lasted until 225,000 miles and had to replace the aftermarket 2x since, one only lasted a year.