Sounds like either your "plastic" shift fork is going or the t-case shift actuator. I wheeled with a buddy who had this issue after wheeling and we knocked on the shift actuator a few times with the handle of a big screwdriver and it shifted back into 4hi. He did have to have the actuator replaced but he was under warranty then. If it is the actuator, it is a simple part swap, don't pay a dealer just order the part online and install it yourself.
If it is the shift fork, I think you are SOL and will have to have a Dealer or trans shop replace it with the stronger metal fork. I have heard of GM picking up aome or all of the labor if you bought the part. Might want to give it a try??
Gone: Mr.Black 08 H3 Alpha OffRoad, Mr.Orange 08 H3 Alpha, Mr.Blue 08 H3 Alpha OffRoad, Mr."T" 09 H3T Adventure STICK
Current: Mr. Red Roc'T 10 H3T Alpha OffRoad / "BILLY" Goat 10 H3 Alpha OffRoad