Last time I replaced a CV on the wife's Subaru, I sprayed some Boeshield T-9 on a few of the rubber parts and was amazed at how it absorbed in and made them look more supple, almost instantly. I sprayed all the rubber bits under the car and they all seemed to regain some life.

I had it around because I use it on my mountain bike, but it was actually developed for aviation (at least that's what they claim).

http://www.boeshield.com/t9info.htm
http://www.boeshield.com/index.htm