Are you sure that your NSBU switch is good? If it is, (your truck is shifting ok) then you will just have to search e-bay, then take it to your dealer for programming. There isn't any cheap way that I know of to do this.
unfortunately it probably does, unless you can live with it that way. There might be place that can repair that, but I don't know how much that would be compared to a new one. How are your batteries? I know they have been know to cause stupid stuff with the cluster when they start going bad.
Same thing is happening to my 2001/2500 HD, gear selector and odometer work some days some not. Some times I'll fire up and its off then down the road it will work again. Shifts good and just changed both batteries. I had heard a rumor that its just a loose wire and is a common problem in the earlier Duramax's. Also heard that it can be fixed with a quick solder. Anyone hear of this issue?????