Keep in mind the cross-section of the Prius is squircular, closer to a rectangle than a circle.
When you conceptualize it as a conic or spherical interface, it does allow for longitudinal rotation but unless your U-joint swivels there're only two degrees of freedom. Rather than a ball-and-socket connection you could use flaps on the top and sides, similar to the flaps on vectored-thrust turbojets. Or big truck's Trailer Tail™.
In another thread I posted this concept truck:
It uses a combination of rigid, hinged panels and stretchable infill.