My Trailblazer goes in and out of 4WD all by itself while I'm driving.

While driving down the highway a couple of weeks ago, my Trailblazer transfer case switch light moved from 2WD to 4WD without anybody touching it. The switch arrow stayed at 2WD, but the indicator light was on above where it says "4WD". When this happened, I could hear the transfer case whining and squealing, which is when I noticed that the indicator light for the transfer case had switched from 2WD to 4WD. I slowed down immediately, but it was SCARY. I had the transfer case replaced 4 years ago at a Chevy dealership at a cost of $2500.00. Needless to say, that "fix" didn't last very long. It was raining at the time of this incident, and the only other time this happened with this vehicle was a couple of months earlier while it was raining, too. What is the cause of this??