Is there a service interval indicated in the owners manuel for the Z-51 E-Diff.?
I don't know what the manual says as I don't yet have my vette, but the diffs on my dodge truck are supposed to have fluid changes every 20k. All the rears I encounter are not sealed, they are vented to account for gas and fluid expansion and contraction, especially in different environmental conditions.
That said, my tri-axle dump trucks run synthetic 80w90 and the mfr suggests 100,000 mile changes. Those rears are also vented, and they see a LOT more dirt than your corvette will ever think about. My opinion, once you have a high quality synthetic in your diff, change it at 5yrs or 100,000 if you are concerned about moisture or dirt contamination. I have the same opinion of transmission fluid for manual transaxles. My Altima that I kept for 200,000 had the trans oil changed once, and it did shift a bit cleaner after the fluid change at 100,000 mi.