4x4 - 2 wheel problems 04 suburban
 

I have a 04 1500 suburban. It has the push buttons on the dash to put in 4 wheel. It is stuck in auto 4x4 mode . It did that by it self, but when i try to get it in two wheel mode it dosent go. If i shut the truck off and press the two wheel button at the same time I start the truck it will go in two wheel and i can drive it. then when I stop and back off the gas petal it is like it is in neutral but the column shifter shows its in drive. So then in order to drive my truck i have to then put it in auto mode. Any advice would be appreciated. Bobby