What do I do if my income was reported incorrectly to Principal?

I work as a medical transcriptionist. I have lupus and have been unable to work since November 20, 2013. Principal is supposed to pay me 60% of my wages while on STD. I have questioned the amount because according to my calculations the amount I am receiving is very low. I was informed by Principal that my employer reported a base hourly rate at 40 hours a week. As a transcriptionist I am paid by production, not by the hourly rate. I receive the hourly rate only when I take vacation or sick days. I am also a team lead and receive a supplemental salary for those duties. This salary was not reported to Principal either. Based on my 2012 W2, there is about a $20,000 difference in what my actual wages are versus what Principal is paying me. Basically my employer has falsified my wages. Is this legal? Can they get by with this or do I have a case to fight? I have called my employer and they will not return my calls.