Summary

This is from http://www.thingiverse.com/thing:35057
The 5mm, 8mm, M3 screw and M3 nut fittings were too tight on the original when printed on my Prusa Mendel.
After trying many things to reduce Z Wobble this was my best option.

My ultimate goal is to try the Zero Z Wobble Upgrade at http://www.thingiverse.com/thing:34140.

Instructions

You may need a drill to make some of the holes. Even though the holes are in the stl file they don't seem to be recognized by my Sprinter.
If anyone can make the holes work consistently, please feel free to make another revision of this excellent coupler.

I also use http://www.thingiverse.com/thing:9864 (mounted at the top only).
The trick is to remove all screws from the Z motor mounts to allow for play. I also tried using Lithium Grease to assist in slippage under the motor and if you must it would be better to use cable-ties to hold the motors down than using the screw mounts.

The trick is to get the z-screw to be loose, not tight. The rods are meant to do the job of keeping things aligned.

Also, the picture is from before using the parts. So I made the parts with Z-Azis wobble. I might post a picture later on the change of results once assembled.

Universal Joint Z coupler, Revised