I have a 2001 Tacoma SR5, TRD off road, automatic, 4x4. While driving, whether in high speeds or low, it tries to go into four wheel drive by itself. It just starts to, but never goes all the way. The four wheel drive light comes on and there is a grinding noise. I have to stop the truck, pull the four wheel drive lever back and manually put it back into two wheel drive for it to stop. Any ideas??