1990 bronco 2 wont shift in to 4x4?

If your 1990 Bronco II won't shift into 4x4, the issue could be related to the transfer case, the shift linkage, or the vacuum actuator. First, check the shift linkage for any obstructions or damage. If the linkage is functioning properly, inspect the transfer case for fluid levels and possible internal issues. Additionally, verify that the vacuum lines connected to the actuator are intact and that the actuator itself is operational.