GSA relative displacement output
Relative displacement outputs of 1D element (DX, DY, DZ, DRx, DRy, DRz based on the local coordinate system of the element). Ie, relative displacement of two points. A rigid-body translation should give zero relative displacement for all components.
Available as member deflections
Neil Howley commented
This would indeed be useful. You should expand on this to give relative displacements for a string of elements.