You can get a continuous rotation servo and it may even be possible to convert the servo you have to continuous rotation. Google should find explanations of how to do it.
You might also investigate sail-winch servos.
However that won't solve the problem of being able to pull the load away from the "winch". If the servo is unpowered the resistance to pulling the load is due to friction in the gear train. With any small motor you will need a similar gear train with the same problem.
A very much larger motor that needs less gear reduction to produce the same torque will probably be easier to "unwind" when de-powered.
But you may also find that the large motor with less gearing winds too quickly.
Is there some reason why you can't run the servo in the other direction when you want to "unwind"?
...R