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The 04 Z06 shocks were designed specifically for an 04 Z06 and work very well on an 01-03 Z06. I'm running them on my 02 Z06. For a non adjustable shock they work great on street, AX, track.
The 06 Z06 shocks were designed for the C6 platform. Sure they work but they were not tuned for a C5.
Depending on your budget and intended use you can determine whether you want to pay the difference in cost. In my opinion the 04 Z06 shocks are well worth it. Even at the current price.
Note: This has been covered many times over. Do a search on 04 Z06 shocks and 06 Z06 shocks in the C5 Tech section and the Track/AX section.