What would cause server leg cramp. Deep in the thigh bone down to back of calve muscles and hurts to walk?

Multiple causes. Can be blood clot, lumbar referred pain, muscle injury, vascular problem, local bone or soft tissue problem . Should be checked out.
Many things. It states that you are taking Methadone ? Why pain as your complaining of could be "sciatica"from a disc irritating a nerve or possibly a deep vein thrombosis.