You have three options:
1) Rent a car.
2) Take a cab (more expensive than 3, below).
3) Hire a towncar from Tiffany towncars. This is what I do. I think it costs $75 round trip, but he'll pick you up at your hotel and be waiting for you at Universal at the end of your day.
4) I have heard that Universal does run some kind of transporation between Disney and its parks. Not sure when or where it departs from. I would contact Universal to find out.