It’s a vacuum operated engagement. Lower driver’s side upper side of front axle. 2 vacuum hoses lead to it from the actuation assemblyman the passenger side fender . Stay outside the vehicle and listen for a click while a second person engages the 4x4 switch on the dash. No click, remove the vacuum switch and bench check it. 12 volts one side or the other, dosen’t matter. Still no click, replace the vacuum switch. If after that and you hear a click, pull one vacuum line at a time and check for leaks. If no leaks, go under the truck and remove the vacuum canister. Clean the plunger, remove and replace the cotter pin. Apply vacuum to the canister to check operation. If no movement replace the cannister (they’re not easy to find). Most of the time all that’s needed is a good cleaning and lubrications of the cannister plunger. Good luck!