There are accessible taxi vans, as the other posters mentioned. The Concierge or Bell Services at your resort can help you arrange for one.
Mears is contracted to provide exclusive service at Disney - including Magical Express and taxis. Mears has accessible vans and Town cars (the Town Cars are not accessible).
Checker Cab, City Cab and Yellow Cab are all divisions of Mears, so you will also see those taxis and accessible taxis.
Most accessible taxis have rear ramps.