06 gmc envoy wont go into 4 wheel drive

[Dopeboy420]

I have an 06 gmc envoy and it won't go into 4 wheel drive. I replaced the 4 wheel drive module on the front differential and I replaced the 4 wheel drive switch in side the truck. I try to change it from 2 wheel to auto or 4 hi or 4 low and the lights flash on the switch but I won't change it stays in 2 wheel drive, any ideas on what to do to fix it.