2006 Lincoln Navigator Shifting during acceleration

I own a 2006 Lincoln Navigator with about 86000 miles on it. V8 Rear wheel drive, or 4x4 or AWD. While jumping on highways or we need to accelerate harder than normal, the truck begins to buck a little. It feels and sounds like it is going back and forth between gears as if it couldn't decide if it wanted to upshift or go back to the previous gear. I don't know if it's a transmission problem or maybe with the fuel injector, not giving the engine enough fuel to accelerate??