Tranny won't shift into reverse! 2003 Explorer

03 Explorer, V8, Automatic. It's happened twice in the last 4 months. I'll hop in my truck to leave my house, shift into neutral, but I don't hear the tranny shift. I step on the gas and hear the engine rev, but I don't go anywhere. After I put it back into park, and then back to reverse again a couple times, it'll finally engage. Is my tranny shot? Any help is appreciated.