Help with 4x4 in my 91 S10 Blazer

You need to be in neutral and not moving to shift into low range

The s10 blazer has a vacuum actuator that sits under the battery tray in those years and goes bad a lot there's a couple ways to fix one which is cheapest is pull and clamp the cable so your front axle is always engaged witch isn't good for on road or get a kit that'll aloe you put a lever on your blazer to engage it or just fix yours witch will probably cost a lot depending if it's the actuator or just a leaky vacuum hose but that is why 4 wheel drive isn't engageing