1984 Ford F-150 Starter problem?

Recently I have had problems starting my truck. It sounds like power is getting to the starter, but that the starter (about half the turns of the key) is not engaging the engine. I'll turn the key and get this loud whirrring sound. It sometimes takes me 4 or 5 turns to even get it to turn over. Sometimes the engine will only crank once or twice and then I get that loud whirring sound.