[ODE] Slider joint without constraining rotation

[Luke Palmer]

I'm trying to control the distance between two bodies without
constraining their rotation with respect to one another.  Right now I'm
doing it by adding proxy masses, i.e.:

    A BallJoint proxy SliderJoint proxy BallJoint B

This seems overly complex to do something so simple.  Plus, I have to
add mass to the system, since 0 mass objects really screw with things.
Is there a better way to do this?

Thanks,
Luke