2001 Jeep Cherokee car wont start

The cars been running fine and 2 night ago when I started it up it turned over but sounded like the battery was dying. I ran it for about an hour to give it sufficient time to charge but the next morning it was worse. Over the next 2 days the problem got worse but the car always started. Last night I took the battery in to be checked and they said it was good. The contacts were kind of corroded so I cleaned them up real well. But this morning it wouldnt start, all the lights came on, the radio and all, I could hear the starter slowly spinning but then slowly stopped after what sounded like a few revolutions. I'm thinking either a soleniod or starter, to me starter doesnt make any sense but i'm open to all advice, thanks