2wd to 4wd can't get back to 2wd

[theodor]

I have a 2002 4wd Suburban that acts up in the following way:

I'll be driving in 2wd and I will depress the 4wd button, it will indicate that I am in 4wd and will act likes it in 4wd. The problem I'm having is once I'm in 4wd I can't get it back to 2wd without pulling to the side of the road, turning the motor off and restarting, then maybe it will go back into 2wd. Sometimes even this may not work. Any ideas what might be causing this issue?