Does Harnessed Darkness proc from twinned Shocks generated by Chain Shock? I'm thinking that instead of Raze, you go Harnessed Darkness, something like this:
Death Field seems like a better option than Wither anyway, due to short cooldown and Wither's limited range.
If you are boss tanking then Wither is far superior, because it has the added advantage of reducing target damage by 5% (huge) and it is a high threat move. If you are PVP tanking then your build gives greater flexibility.
Either way, Harnessed Darkness is not good for PVE tanking because you will never use force lightning.