Toyota Repair: Toyota pickup 4WD inop, vacuum hoses, drive shaft

Question
We have a 1994 Toyota pickup, V6, 5-spd manual, Auto locking front hubs. When the transfer is shifted into 4WD, the indicator lights do not come on and the hubs do not lock.  The front drive shaft turns as it should.  Can you suggest possible cause(s) and remedies from most likely to least likely order. Vehicle has had recent extensive engine work but no transmission, transfer or drive train repairs. Thanks, Brad

Answer
There are two main causes for this, the front differential uses a vacuum operated actuator to engage it to provide front wheel drive, the vacuum hoses for the actuator may be disconnected either at the actuator or at the vacuum operated solenoid under the hood on the right front fender, the two steel lines on the actuator are clogged with rust or the actuator diaphragm is leaking, it may be necessary to remove the actuator and test it by applying vacuum to the ports.