Plot differential displacement between two points, parametrized by z-coordinate

I'm trying to plot the differential displacement u, between points on opposing faces of my model but parametrized by the Z-coordinate. I can easily generate the data with a parametrized curve along each of the faces individually, but for the life of me I can't figure out how to compute their difference!

I'm sure there is something trivial that I'm missing on a conceptual level, but it's been a frustrating morning :/ Any help would be much appreciated!

Thanks,
Neal