Transport America terminated my employment because my medical leave with them expired. Are they in the wrong??

Under the FMLA, many employees (if their employer is subject to FMLA requirements) are entitled to up to 12 weeks of medical leave for treatment of their own serious health condition.  If the medical condition is diagnosed and can be treated in a way which permits the employee to return to week at a "date certain" sometime shortly after the 12 weeks of FMLA leave expires, the courts may require the employer to extend the leave (protect the employee's employment) for some reasonable additional period as an "accommodation" for the person's disability.  In your case, however, the leave period has expired, you have no diagnosis, your doctor will not release you to return to work, you cannot drive, and your employer has no idea when you might be able to return to work, if ever.  Under those circumstances your employer is probably entitled to terminate your employment and replace you.