Constrain single axis rotation...

Okay, I have two objects.  What I would like to do is constrain the rotation of the second object's z-axis to the rotation of the first's z-axis. 

This would mean that the second object will only match the rotation of the first's when it's z-rotation changes, but ignore any other changes.  This will mean the second object will need to be able to translate the movement of the first object when they are not aligned. 

The two objects are also pivited slightly in the world and when I apply a wire parameter, it gives out odd results, the second object tends to rotate about on serveral different axis.

Now I can make this work reasonably well when the objects are situated flat in the scene, but if the objects are offset when I start, I start having lots of fun and the second object tends to wonder around in a fit.

What I would like to know is how to do I translate the rotation of the first object so that I can isolate the the z-transformation and then apply it to the second, even when the objects are offset slightly.

Cheers
Shane