I have a 2019 Duramax that keeps showing SERVICE 4WD on the display during heavy acceleration in 2wd but no codes, I took it to the dealer and they said it was a fluke because there was no code, I bought it new and have only used the 4wd maybe 5 times ever, I tried it yesterday when I was in some deep gravel and it wouldn't go into 4wd, what is going on, it can't be the actuator could it?