I have a '99 4Runner and at first it was stuck in 4WD. After fixing this, it now will not engage in 4WD. My 4WD light flickers 24/7 and when I push the 4WD button it turns off for a few seconds and then comes back on. I have tried shifting to 4h from stop and trans in neutral. Then driving forward a few feet. Stopping. Then driving in reverse a few feet. I also replaced front diff. switch, put in a new transfer case, tested 2 of the switches on the ADD for continuity(both worked as they should), and cleaned out my vacuum lines. But the light continues to blink.

 

https://www.yotatech.com/forums/f2/4...er-ltd-131201/

The information in this thread will help you out.