Blazer S10 Jimmy 4WD Vacuum Troubleshoot - 4 Push-Button

Not noted in video:
If you don't have symptoms noted for a vacuum leak, it may be the control module. Before replacing: remove the electronics connector, clean it, dielectric grease it, plug it back in. See if that does the trick. Possibly clean the push-button switch connector itself as well; it may need to be replaced or reflowed.

If you have a 3 button 4WD, there is likely a vacuum switch on top of the transfer case that is easy to replace. It often fails.

Finally, a vacuum leak can be caused by a few other things but they may not be detrimental enough to cause the 4wd to not engage. Additionally, if you continue to run lean or something after fixing the leak and clearing the codes (or you can just tell by driving), it may be an intake failure. These engines are notorious for having bad seals/bad gaskets on the upper and especially lower intake manifolds. Look to see if you're dripping coolant.