is your Z71 a Silverado (new body style in 1999)? If it is, it's likely not the switch.

If you have the 4 button switch with the round AWD button, you most likely need the actuator position sensor replaced. It's quite common for those to fail by rubbing through the resistive coating on the ring, thus causng a voltage error and throwing a switch code. There's also a programming update available.

Don't program it without replacing the sensor.

1990 Montero RS (In pieces... for now)

KG6VNX