The thermal linear actuator goes bad on those with some frequency. It is mounted on the IFS differential housing (I think on the pass side). Check with a voltmeter.
There exists aftermarket solutions, but I don't recall exactly what right now... (ETA:
one solution: Posi-Lok)
If it is a four door pickup, I think it has a "solid" axle and not IFS. In that case, forget what I just said...
ETA: More info from
HERE:
There is a TSB about these actuators. when working properly they take about 10 seconds to engage. lots of complaints back in the day. GM makes a motor and an adapter harness that goes for like 230 out the door. It engages the 4x4 insantly like an old setup. far superior to the thermal actuator. if you do alot with your 4x4, you might want to consider it.