You can do some verification with a driveway test with the right front off the ground.
place in 4wd. leave in park. You must start the truck to activate the vacuum.

Turn the driveshaft by hand.
Front driveshaft will not turn(engaged). TCase is working.

Turn the r-front tire by hand. if the driveshaft turns then the tcase is not engaging. If the driveshaft does not then the vacuum is not engauging the couple of the r-front to the diff.

Pull the hoses from the vacuum pot on the front axle. Then and have someone start the truck and switch from 2wd to 4wd while you listen and feel for the vacuum to change from one line to the other. If there is vacuum then look for the actuator to move when hooked back up.

A control issue like nobow mentioned might disable any of this from happening. but hopefully it will help from a mechanical view.