Either the wheel cylinder for the drum brakes busted and is leaking brake fluid, or you have an axle shaft seal leak leaking gear oil.
Pull the drum off and see if you can figure it out. If it smells awful and strong, it's gear oil. If it's gear oil you will need to remove the diff cover, undo the bolt and pin that lets you pull the axle shaft out, then yank the old seal and install a new one. Probably a good idea to do the wheel bearing at the same time if you end up needing to do all that.