I just changed out my NSBU switch, due to SERVICE BRAKE SYSTEM, irractic shifting, and harsh engagement that would not give me reverse or maybe forward in 3rd.
When I did the change and restarted the truck, no difference. Still had all the same symptoms.
I called my mechanic friend who told me that is normal and that I would have to clear the codes in order for it to reset itself.
I will find out tomorrow. I am in a race against time. I need the truck to get my car to the track this Friday for the Pinks Arm Drop LIve.
I hope he is right!!
Matt.

You will have to clear the codes in the TCM with a Tech 2, EFI Live, or a really good scanner if the NSBU switch was your issue. From the sounds of it, the truck diagnosed itself (hence limp mode) and set TCM codes.

Thanks Brad. I am keeping my fingers crossed on this. We have a mechanic at our company who has a GM scanner. He let me use it before. I am sure he will again.
If this clears my problems, I will be giving my Allison tranny a fluid change with some good old Castrol tranny oil.
Thanks again.
Regards.

You can also try disconnecting your batteries for 10 minutes and re-connecting. This clears codes as well.

I disconnected the batteries for 3 hours. Still, same thing.
I don't know what to do next.