Not sure why you want to do this, but at high speed this thing will be out of control. Then the slightest turn of the wheels will make the truck go topside, and it would be a pain in the buttox to try and line up the two axles straight. That being said, if you still want to do it...
That RB Innovations kit is crazy expensive, 130$ for a piece of metal it's insane. Here's an idea for you, might involve a bit of DIY, but nothing extravagant.
Look at your steering setup in the front. All you need to do is recreate the same setup in the back, and use a Y connector for your new servo in the back. You can buy a cam servo saver kit for 15$ at Amain hobbies
http://www.amainhobbies.com/product_info.php/products_id/25549. Add a few bucks for the rod and wire, and a couple pieces of metal, to hold all that in place and that should be it. I don't think I would try with one servo, as it might hinder the performance. Yes you could buy a crazy strong servo for big bucks, but that would defy the purpose of DIY.
Here are some pics of a guy who did it:
http://www.freewebs.com/hpi_savage/4wspics.htm
If you want a kit I think New Era also had one a while ago, not sure if they still sell it though, and I don't think I would buy New Era to be honest
For sure there are not a whole lot of kits for this out there...and for a good reason: it's just not that good of an idea.
Hope this helps!
