Why does 2000 Toyota forerunner go into four-wheel-drive while driving in 2wd

What is your question?

When it drops to forward Drive and gives me a solid green forward Drive light on the dash..i've checked the vacuum lines and changed the transfer case switch on front differential. If I am plug one of the sensors on the transfer case it will switch into 2 Wheel drive for a little while but then it drops back into

What makes this problem better or worse?

The sensor or switch on top the transfer case towards the front of the vehicle with a wire coming out of it that leads to a plug, when I unplug it my dash light goes out and 2 Will Dr. works fine for a while then it drops back into four-wheel-drive while I'm driving it