Sounds to me like its the steering wheel motion sensor, it doesn't sound like its a problem with the steering components themselves. This is a fairly easy and inexpensive fix. if you disconnect the steering assist control solenoid you should find the problem goes away, this all depends on whether your Sierra has VES (variable effort steering) which I'm 99% sure it does. Again this is only a guess based on your description. You can always check all the steering linkage, hubs, tires, etc. for free to rule out that as a cause.
The VES is up toward the top of the steering column if i recall correctly. It's has been years since I went had to replace one.