I noticed something about your question, you said the “service four wheel drive” message stays on when you turn on the ignition. Further, you noted you swapped out the selector switch, the encoder, the fuses and harnesses. And, you finally noticed that “lights do flash” when you turn on the truck.

Have you ever thought of looking at the flashes, copying them down and then checking with a Silverado service manual to see exactly what’s going on. You see, there’s a whole piece of information — rather huge — that you haven’t even begun to look at, the codes contained in the pickup’s onboard computer system.

Now, it is great that you have swapped out all of the relatively easy stuff. But, it doesn’t have anything to do with finding the real issue with your truck. That information is contained in the codes that are flashing. Indeed, you should have figured that something else was wrong with your truck when the lights kept on flashing. Your truck was trying to tell you it had a problem and it was doing its best to let you know it in real-time. (Every car or truck on the road today has several ways to read the fault codes that are generated and stored. The most basic is the flashing lights that you described, while the most advanced is the Chevrolet-approved diagnostic scanner.)

So, here’s what I suggest you do: either obtain an OBD-II scanner (they are available fairly inexpensively) and insert it in the test just under the front left panel of your truck). If you get a generic scanner, then the report you will receive will be generic. Still, it’s better than no information at all because, though the answers and codes will be generic, they will still point you in the direction you must take. I think, though, that at this point, it’s a good idea for you to invest in a Chevy-approved OBD-II scanner so that you can see the Chevy codes that have been generated and use them as your diagnostic roadmap. Let me know what happens as I am definitely interested.