I pulled apart an xb2 ball diff just to see it and holy cow the rings are thick. I'm pretty sure that the outdrives will wear faster than the diff itself. In my Associated cars I run tungsten balls and ceramic caged thrust bearings and I have a diff in a B5M that is over a year old....I just recently replaced the outdrives and regreased it, didn't replace anything. I'm going to go out on a limb and say that almost all diff failures are due to the thrust bearing failing. Its interesting that XRay uses a caged thrust. If you look at it from a quality perspective the cost of an xray diff is pretty close to that of an associated or kyosho if you are prone to upgrading the diff internals. Xray is top quality. That being said, its still hard to justify the cost of the parts. The kits are reasonable, especially if you consider that they need very little in terms of upgrades. If you buy an RB6 you you have to spend a mandatory $100 on stuff.