Caltrac operation on leaf springs

The shackle still has to move because as the suspension goes through its travel the spring changes length. So it has to move.

FWIW, the faster guys run sliders back there. They usually weld into the frame rail and replace the shackle. Pretty cool but I'm not doing it on my car because I don't want to change the car any more than I already have.

So yes, the shackle has to pivot. Or you'll have binding. Like trying to run a ladder bar without a housing floater. The bars and springs have a different length so they move through different arcs. The floater allows the two different length parts to move in different arcs without binding.