if you are not opposed to riding pubic transportation their is the LYNX bus system. they have a vast bus network and i am almost positive you could take it from the Disney transportation center to universal. check out their webpage
Also you could even rent a car for one day, we quoted that one time and the cost was around $40, and I believe Budget will pick you up and take you from your hotel to the rental center and back when you return the car.
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