Had the same on a helion dominus,
got bored, threw a 5kv and wp8 in it, 3s, and took it out, first few runs were immense, then grinding under acceleration. Tried everything you did, then pulled front diff out, still noisy, swapped rear diff with the front one, noise gone,
ordered a new complete diff and centre shaft gear, fitted and all is fine, just sticking to 2s from now on though.
rear diff looked okay visually, but clearly wasn't.