I'm assuming that you're not renting a car?
I would think the quickest, cheapest way would be a cab. It's a pretty short ride from Universal to Disney World.
If you want the option of a grocery stop, take a town car that offers a grocery stop.
I might even look into renting a car for just one night. I did that once. We wanted to make a day trip to universal, so we rented a car. We picked it up at the airport, used it that day, and the next day, dropped it off at the Doubletree hotel at Downtown Disney. It was about $70, but it was worth it to be able to go somewhere outside of Disney for a cheap dinner, avoid the shuttle hassles, go to the grocery store, so a little exploring outside of Disney...... I really enjoyed having a car.