The following should work. Basically, use ndgrid (or meshgrid) and sub2ind to compute all your indices at once:
[direct, indirect] = ndgrid(1:34, ind(1:34));
index1 = sub2ind(size(z), direct, indirect));
index2 = sub2ind(size(z), indirect, direct));
s = abs(z(index1) - z(index2)) ./ (z(index1) + z(index2));
I'm calculating the whole matrix at once instead of just the lower triangle as that will be faster anyway than flipping and copying the lower triangle.