How can you get to Universal Studios from Disney World without a car?

I am currently planning a trip with some friends to Orlando. We were planning to fly down, visit Disney world for 3-4 days, and then spend 1 day at universal studios to see the new Harry Potter park. We are all either 19 or 20 and just barely too young to get a rental car. We were thinking about staying at either a Disney resort hotel or a hotel on disney property, so we would have a shuttle to take us to the parks, however we don't know how we can get to Universal without a car. Do you know of any shuttle or bus system, or etc. to get us from park to park? We would prefer not to use a taxi and free transportation would be great, though not a requirement. Thanks!