4wd problem

I have a 96' Tahoe w/147k and electronic shift 4x4. When I switch into 4x4 it switches fine, but when trying to go back it stays in 4x4. The 2x4 button lights up, but it still stays in 4 wheel drive. If I let the truck sit for a while and then turn it over it has switched back into 2 wheel drive. Any ideas??? THANKS

Are you using 4 wheel on dry payment?
Are all tires and wheels matched?
Try stopping and put transmission in neutral or back up just a little.
Is it the one with auto 4W button on dash?

I ran the truck on light dirt and I have two different pairs of tires. I was told it could be a thermal actuator, but Im not sure what that is or where it is located? thanks

Have you put it in neutral or backed up a little?
What transfer case do you have?