Four Wheel Drive Not Working

I have a 97 4Runner, which has been virtually trouble free until now. It won't go into 4 Wheel Drive. I can "shift" it into 4H and 4L position, but it doesn't actually engage. The indicator light does not come on when I shift it...which I have heard is somehow a good sign. Any thoughts, diagnosis or suggestions would be appreciated.

Your ADD actuator is not functioning. The cheapest possible fix is to change the front differential lube. Use 75W90 gear oil. If that doesn't solve the problem, it will get more complicated.

I am having the same issue right now, and have also found that vacuum hoses for the actuator air bags may also cause this issue.

Leaking vacuum hoses can cause the ADD actuator to not function.

(air bags?)

yeah, air bags actually, kind of like balloons that blow up to force the actuator to move over the shaft and lock in the 4 wheel.