I am a member of Scikotics as well and Strup sent out a flyer to members just before Christmas. Various discounts on items for year end clearance. I think it was 50% off retail. I don't remember the exact price.
I called and talked to Charlie himself to confirm the offer and then ordered it via the web site and pay pal. (I ordered some other stuff too, which is why I can't remember the exact price of the axle back itself.)
Most people I have talked to e-mail Strup to get a price and availability on what they want and mention the clubs/forums they are affiliated with at that time. Charlie has a reputation for treating people right and responding quickly to inquiries.
If I remember correctly Yellosub on this forum was in on the initial run/prototyping of the xD axle back and might have more info. There is a thread on here with a video and sound clip.
Here is the forum link:
http://www.scionxdforum.com/forum/showthread.php?t=723