2003 Ford Explorer Leaking Fluid

My Explorer has leaked here and there from the rear of the vehicle for a little over a year. I had a friend of a friend who's a mechanic take a look a year or so ago and he told me it was nothing I needed to worry about right away. Well, now it's leaking a little more consistently (instead of a drop or two a week, it's a drop or two a day). Can you tell me what this is and what it might cost to repair?