I have a 1997 4runner 3.4v6 automatic with push button on tcase shifter. Front diff will not engage and no light comes on the dash when shifting transfer case or pressing button. Everything appears to be working fine but I can't figure out why the diff won't engage. I replaced the 4wd computer but it still won't work and there's still no light coming on. Any suggestions on what the problem could be?

 

Here is the issue I had with my 4WD... Every so often I would engage the 4WD to give it a little exercise. At about 200k miles I started noticing a delayed response from the 4WD. This wasn't an issue until one day when it would not disengage. At first I thought it might be a vacuum line, but I could not find any leak. Eventually I found a thread describing a similar issue with a 4WD position sensor. The sensor is located on the back of the front differential. Oil from a leaking valve cover gasket had run down the engine and caused the position sensor to fail. I removed the sensor and gave it a good cleaning with break parts cleaner and it solved the problem. Hope this helps!