You have a couple of options.
1.) A cab - this is probably the easiest way, although probably not the cheapest. I'm not sure what the fare would be. I'm going to move this to the transportation board, so maybe those folks can help you.
2.) MEARS - call them and arrange for shuttle transportation. I believe it's $12 per person for roundtrip transportation from Disney to Universal. You need to call an hour (I think an hour - might be more) before you're ready to leave. They then come and get you and take you back to the hotel.
3.) Lynx - there's a LONG drawn out scheme of connections that you can make. This is the cheapest - however, the most complicated and time consuming way of getting there.
4.) Rental car - what I've done in the past is get a car for maybe one or two nights.
Budget does one way rentals, and I think National does too. There is a budget location at the Doubletree on Hotel Plaza Boulevard, and a Dollar at the Holiday Inn on Hotel Plaza Boulevard. There are also National locations at both the Car Care Center outside of the Magic Kingdom parking lot, and the Dolphin.
Good luck, and have a fantastic trip!