transfer case Grinding them slamming

My 4 wheel Drive works fine but when I take it out of 4hi
I will be driving for a while and then I can here the transfer case (I think it's the transfer case) kinda grinding/moaning then I have to slow to almost a complete stop and then my truck shutters and somthing slam in or out what ever it is doing..

I was told by several people that it is the sensor/switch on the tranfer case !!! Is this true

And I put it on order at the chevy dealer and it is taking over a month to come in still nothing yet
Does anyone know where I can get this part online/other ????

Thanks :smokin: