The G-parts from the VW Touareg has two plastic pieces that replace the spider gears to lock the rear diff. They are on Tamiya part number 9005422 and should be available from Tamiya.
http://www.tamiyausa.com/product/ite...uct-id=9005422
Its nice that it is a reversable mod. But I don't think that it is TCS legal. Take a look at the exploded view of the Touareg and you can see them as G-5 near the rear diff.
http://www.tamiyausa.com/pdf/58324e.pdf