Based on a few views / user feedbacks here & there those cvds don't seem all that great..
I would go for the summit driveshafts ( you need the longer side ones ) as an upgrade over the stock driveshafts, or look into the LST cvd mod:
http://traxxas.com/forums/showthread...=1#post4633281
http://traxxas.com/forums/showthread...=1#post4864749
you can still view the trx forums, just make sure you're logged out.