The day when Universal offers some kind of shuttle back and forth to Disney, is the day the flood gates open. Disney doesn't want people going back and forth between the parks. If you saw the latest ticket overhaul in the news last week, the best deals happen at 5, 6, and 7 days. Disney wants everyone to stay on their property the entire week.
If you're not renting a car, then it's going to be the limo services. You could take the #50 bus to SWF and back, but going to USF involves multiple busses and it would take too long.
I tell people to rent the car, it's worth it in the long run.