2000 chevy s10 blazer

Check the vacuum switch under the hood in the center of the firewall...follow vac diag

If you have been in a lot of mud it can get caked up in your linkage and keep it from working properly
hope this helps

think your starter motor is no good or the battery is flat ,try charging the battery for a couple of days as well

check the fuses supplying the front viscous-coupling system solenoids, there should be at least one fuse for this marked AWD or 4WD. Also the locking hubs or the front differential may be damaged from engaging 4x4 while moving.

If it is an auto then the fuse or box fluid levels are likely areas to check.

If the fuse is found to be intact, check the relays click when 4x4 is selected, and replace the necessary relay if it does not operate.

You will need the service manual for your car to locate and identify the correct electrical equipment for the 4x4 function.

Change shift solenoid

Do you have power with a test light in and out of the switch? Do the turn signals work? Will the brake lights work if you play with the turn signal handle?

go to autozone.com, enter your vehicle specs, and follow the prompts for the repair section, then to electrical diagrams..........and its FREE!

You may contact a GM dealer for assitance in getting a manual for that year and model.
I would also recommend getting a good repair manual for that vehicle. The savings for one repair will more than offset the price of the repair book. I use manuals produced by Haynes which can be found in most auto supply stores like Advanced Automotive or Auto Zone.

Most common problem is the vacuum diaphragm that is so inconviently mounted UNDER the battery tray.The cable that locks in the front diff is attached to it and vacuum pulls it in/out.

Had the same problem, disconnected the battery for about fifteen minutes, reconnected, 4 wheel drive hi and lo work like a charm now.