03 shifts into 4wd hi on it's own at speed (dial selector version)

[PathyPop]

About a month ago my son's 03 LE with the dial selector 4wd started shifting into 4wd high while moving, even on the freeway at 65 mph. He always drives in 2wd and cycling from 2wd to 4wd and back does nothing when the problem occurs. It always goes back to 2wd when he parks it for awhile, probably because he shifts to park or reverse. But then it acts up again when he is at speed. I checked the wires on the dial selector and they are fine. I also changed out the transfer case fluid and checked connections but haven't replaced any parts as I'm not sure which would cause the problem.

Has anyone had this problem? I'm figuring its a switch, solenoid or servo but searching around I can't see what I should replace. Thanks in advance.