my brother has an 03 chevy half ton with the 6.0 litre engine. When you push the button on the dash to put it in four low the truck sounds like it is engaging but actually goes into neutral somehow. Also just driving down the road every once in a while it will go into four high on its own. Does anyone know what the problem is here?

does it for sure go into 4hi or does the light just say its in 4hi. My encoder motor on the tcase went out and driving down the road it would say I was in 4lo and that was for sure not right. Then it wouldnt engage when I pushed the button for what ever I wanted it was just stuck in 2wd pretty much but would say it was in neutral, 4hi, or 4lo. It was just all messed up.