Search "top photon" in the TC4 thread. The diff is discontinued but I recently saw one for sale in the classifieds. Requires an extensive amount of modification though. I have one car with the conversion and it really isn't much of an advantage over the ball diffs. In fact, it think the lightweight plastic diffs work better. They just don't hold up very well. Here's a post with a picture of the conversion.

https://www.rctech.net/forum/showpos...ostcount=11741