Being a jeeper when younger, and using a winch a lot, I can not imagine running a winch on just a battery (I would want a real good battery and a high output charging system). A low battery will cause your winch motor to heat up and also shorten its life. As you stated, you don't want to get half-loaded and run out of juice for your winch! Then you are stuck with your load half on, and half off and no winch!
I agree with Bartamos, it may be a simpler and better over-all solution to change your truck over to 12volts.
