Been there, done that, and the wave has long since passed me by.
While I believe there is much to improve on a Series or Defender in terms of reliability and durability, I see no reason to overbuild the rig for the intended task. Add in a margin of safety, for sure, but not build something for rock racing when all that's required is a reliable overland vehicle.
I realize that some of the advice may seem like it contradicts itself, but actually is pretty linear from my (skewed) view.
- You can run the stock axles/carrier, but you certainly would want to carry spare shafts and be otherwise prepared for occasional breakage.
- You can upgrade to an ARB with the stock shafts, but carry multiple sets of axle shafts.
- You can upgrade the carrier (open) and shafts to 24 spline, and it's doubtful you will break anything with overlanding use.
- You can install a rear ARB and upgrade the rest to 24 spline, and probably not break anything with overlanding use/moderate wheeling.
- You can install ARBs front and rear with 24 spline shafts, and it's possible that you could break a ring/pinion (a lot depends on the ratio/quality of gearset and how the lockers are used) in more rigorous wheeling.
- You can install Toyota 3rd members (open or e-locker) and be pretty darn secure up to hardcore rockcrawling.
Figure out where you fit in that scale, and you should be good.