I went on short term disability to deal with an injury. now the doctor is not clearing me to go back to work at neither my normal job or modified work. am I allowed to quit to pursue other jobs and if I do am I entitled to severance oay

You are not legally required to follow your doctor's advise. However, your employer can refuse to allow you to return to work because your doctor says you are not capable of doing so. If you choose to resign and find another job, you would not be entitled to a severance package.