I had that happen to me a few times with my 07 4.8. I would have the engine decrease rpms and power, and then an AWD error sign come on. I had to pull over and stop and restart the engine, which would wipe it the error messages off, and things would go back to normal. After it happened a couple of times, I took it in to the dealer, but they could not find anything wrong. Anyway, it happened again, recently, and all sorts of lights went off, including a message saying it could not be driven safely etc. It worked well enough to get it to the dealer using back roads just in case, and I was told the wiring network was not communicating properly and sending error signals between components of the engine and transmission.

Basically the "wiring harness" was faulty. After a week of waiting for the part, I will pick it up again soon. Not a pleasant experience, and in my opinion, rather dangerous. It usually happens at relatively low speeds, 40-50mph and half the time, going uphill. Hope this fix is final...