01 tahoe "Service 4wd" light turning on.

[MAC67]

Hey Guys . Ive been looking around on this problem and its pretty hard to narrow it down , Seems like everyone has differnt symptoms. Its a blizzard outside and a sunday afternoon so im just trying to get ideas.

Wifes truck , she just bought it with 134,000 on it. Runs down the road fine , lights ont he switch all work good. Went to go get grocerys thismorning and i hit the 4 hi " first time ive tried it since we bought it" it went right into 4 high just fine , i made it 2 miles down the road and the service 4wd light came on .
It would not let me shift back into 2hi until i shut the truck off . So i pull off to the side of the road , turn the key off , back on and boom she shifts fine back and fourth 2 hi to 4 hi but when i get to going down the road in 4 hi it will come back on and lock me into 4 hi ,
I can shut the truck off and put it back into 2hi . I know the 4wd is working , No wierd sounds aside from the normal front differential hum.

Any ideas ? could this be the switch even tho it is working ?