[petek]

I would do it with a tangent constraint between the ball and socket surfaces and an anglular constraint between planes or axes on the ball and the socket. I played around with a ball joint model last night and found this to work as long as you don't need more than one degree of freedom (motion, as in constraint-driven animation). When I tried to allow rotation around the ball and yaw side-to-side, it would usually flip orientation at some point.