I know Kyosho also makes titanium cross-shafts that will work with the Kyosho/HotBodies/Jammin cases and gears we've been talking about.
I'm not sure if you will see a noticeable difference in titanium over steel, and I think the titanium ones are pretty expensive. I am actually using the Ofna aluminum cross-shafts and they seem to hold up fine. Again, this is in a 1/8 buggy, so the Savage will be tougher on them, but I think steel or aluminum is fine (probably steel for the Savage?).
EDIT: Here are the Kyosho titanium shafts, at $15/pair you're looking at $45 for a set of 3 diffs. I'd just go for steel:
http://www.amainhobbies.com/product_...el-Shaft-Set-2