I was having issues with the high/low byte as well, turned out I found the worlds worst example of interfacing the SD21 and Arduino.
If I could remember where I found it I would post a warning. This post and quick test program had my servos working perfectly, in about 15 minutes, well except for speed but I can figure that out.
I am trying to build a graceful start and end leg positioning for a hexapod, basically an unfold from storage to start and a refold to finish.
Servo speed is a key component, as well as battery life to know when to shutdown once it is autonomous.