bbk186, it does sound like spondylolisthesis. It can be missed in stationary imaging like a CT or MRI if the movement of the vertebra is dynamic. An anterior and posterior x-ray could be revealing.
You might see a chiropractor for the active physical exam they are amazing with the relationship between joints and muscle.
The most common cause of pain from the knee to the ankle in lumbar spondylolisthesis is nerve root compression of the L5 spinal nerve root due to the forward slipping of a vertebra in the lower back
I have no medical background and don't have the kind of pain you are experiencing.
https://www.precisionhealth.com.au/healthcare-services/pain-management/conditions-treated/spinal-conditions/spondylolisthesis/