Depending how long you are talking about, it might be cheaper to rent a car through hotwire.com within a couple of weeks or a month of leaving for your trip. You can either rent it from the airport or from one of the companies nearby Universal once you are done there. There is a $15 per night parking charge at Universal which you could save by getting a car at the end of the Universal portion of your trip.
One of our trips, we rented a car midway through our trip by taking a $10 taxi ride to a Budget car rental site and then my hubby came and picked the rest of us up. We returned it after just a few days and used the Magical Express to get us to the airport. We got to do a little shopping and had the convenience of a car for a few days as well.
Good luck!