[Mikado14]

Better check that hose. There are three hoses: Vacuum supply that comes from a T in the area of the distributor, Vacuum line to the front actuator, Vent that goes to the firewall near the heater box side with a vent cap on top. These three hoses are usually bundled together in the transfer case area and terminate into a rubber plug that is keyed to the top of the actuator switch.

You will also find a transfer case vent hose in the same area. Also, and I might be wrong in this case but I believe there is also a vent hose from the top of the tranny.

IMHO, your actuator switch that activates the front axle (the one with three vacuum lines) may be stuck. When you activate the switch to return to 2WD, the front axle is not disengaging and the switch on the front axle is sending a signal that it is engaged, thus the flashing light telling you that there is a problem.