Since no one has mentioned it, and I have had this issue several times. The first time I had an issue with it, I replaced the switch on the side of the transfer case and that took care of it. The switch is what activates the solenoids that shift the fork in the front diff and enables the front axle to be engaged. I would suggest checking the switch with a multi-meter. IT is located where it can get all kinds of gunk into the wiring connector and not get a good signal to allow the solenoids to work. The last time mine acted up, which was last year, I took the wiring connector apart, cleaned each side of the connector with electrical contact cleaner and added some "Di-electric grease" to each side, then reconnected the wires. It worked flawlessy the rest of last year. I think that is why Toyota recommends that you engage the transfer case and 4 wheel drive at least once a month. Which reminds me that maybe I should check mine out, since I haven't for quite some time.

One more item,is the fact there is also another switch on the front differential. This switch is the one that actually causes the light on the dash to come on, but I have never had a problem with that one. I have checked the operation of it though and it has always worked, as long as the fork was shifting, it was OK.

I'll see if I can find the article about disconnecting the ADD feature and post a link. Here it is: http://www.4x4wire.com/toyota/tech/add/
Another link that might help would be the one that has the '93 Toy Truck manual on it. It would give a breakdown of the ADD system and should show the location of all of the components. All T's should have the ADD system that was in use on the runners and small trucks.
Here's the link for the FSM for '93 Toyota truck: http://personal.utulsa.edu/~nathan-buchanan/93fsm/

If you disconnect the ADD, then all you have to do is lock the hubs (if you have lock-out hubs) and pull the lever back.