I have a 94 s-10 blazer 4.3 vortec. It has the push toggle on the upper left of the drivers side of the dash, to engage the 4x4 high or 4x4 low while driving. It worked for a few years after I bought the truck lightly used at 84,000 in 2004. After that the 4-high or 4-low will only engage if the truck is in gear and stoped in R, OD, D, 3, 2, or 1 st, and to get to 4-high you have to select 4-low first and both indicator lights are on. I have replaced the 4-high actuator that is below the battery, replaced the rotted vacume line right by the cap and rotor and it is still not working properly. Is there any thing else I may be missing here?

 

You may wish to scrap all that crap and add in a cable to manually engage it yourself. It scraps all the electronic junk, and lets you pull a cable to engage the front axle. I don't know what its called but its a reliable mod that a lot of guys do.

 

I hope you never engaged 4 low when you were driving.