06 silverado excab 4x4 5.3 no mods 110k. Driving down the road when service 4x4 comes on, then moments latter service air bags. Truck feels like it went into limp mode. Fuel mileage goes to crap and starts shifting hard. I'm at whits end trying to find out what's up code scanner only says P2771 and that's it. Any help would be much appreciated.