Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
I'd check the front differential actuator, located on the passenger side inner fender well in many applications under the battery. These are typically vacuum units and are prone to failure. With the engine running, and shifted into 4wd, check the vacuum to the actuator, as well as if it's functioning.
Let me know if you need more help/
need help on a 1999 Chevy K3500 pickup with a 7.4 L