Toyota Repair: Shift Selector Sensor (4runner 4x4), vacuum pipes, vacuum hose

Question
I have a 92 4Runner, v-6, auto transmission, 4x4, auto locking hubs. The shifter for the 4x4 is just to the left of the Auto shifter on the center console. It looks just like the shift lever for the transmission. Sometimes when I shift into 4x4, it doesn't engage the front wheels. The 4x4 light on the dash does NOT come on. However the front drive shaft WILL turn but not engage the wheels. Then all of a sudden it will start to work again, maybe an hour, maybe a day, maybe the next trip out. Starts and stops for no apparant reason. Some people have mentioned a "shift selector sensor" and say it can get dirty and not work properly. Have you heard of this. Where is this sensor? If I were to install manual locking hubs such as Warn (brand) would this solve the problem of this dirty sensor? (if that's the problem).
Thanks
Frank
Frankjake@gmail.com

Answer
The problem you are having is with the Automatic Disconnect Differential(ADD) actuator located on the front differential, it is supposed to engage the front differential to provide 4 wheel drive, this actuator is vacuum operated from a vacuum switching valve, the most common problem is that a vacuum hose may have become disconnected, another problem may be that the actuator vacuum pipes may have gotten clogged up with rust or other foreing matter, the 4x4 indicator switch that turns the light on the dash on is directly connected to the front differential so if the actuator does not engage the light will not turn on and there is no 4 wheel drive, I know because this happened to my 4-Runner and I got stuck in deep sand but that's another story.
The best advice I can give you is to replace the actuator, manual locking hubs, as far as I know, can't be used because the front differential is locked by the sliding gear in the differential and not on the hubs.