06 GMC Sierra 4 wheel drive slips out of gear

My GMC Sierra 4 wheel drive while it is in 2 wheel drive push button selector will slip out of gear completely. I have to turn off the engine to be able to push the button for 2 wheel drive or even be able to put it in park. If it were to pop out while just parked. It would roll off. I have replaced the electronic selector switch in the dash. It did not change anything. I pulled the front wheel drive motor out and plugged it back in and tested it to see if it operated. It did not move. Can you help with this problem? Thank You