Would sacrolilitis or SI joint dysfuncation cause sciatica? Or is Piriformis syndrome more likely, Had MRI of spine and no disk problems found.

Yes . An abnormality of the SI joint such as an inflammatory condition, for example can cause a "sciatica". This is because the L5 nerve, which forms part of the sciatic nerve, runs right over the SI joint. Piriformis syndrome is another example of a condition that is not spine related but can cause sciatica. A detailed exam can usually destinguish the cause.