2001 chev 2500hd 4x4 My truck started switching into neutral from 2hi when I stop. it don't do it all the time then I have to go to 4hi the back to 2hi then it will move. The 4wd light will come on some times. I have drained the transfer case fluid, replaced the switch and cleaned the grounds under the truck. Yesterday the light came back on but it did not switch out of 2hi. What can it be