Geppo,
Since your PD is 30 days older that the current processing time, you can call the 800 number and request an inquiry. It is possible that your I-751 has been transferred to your local office. If that is the case, then the service center timelines no longer apply. Just remember, you can file N-400 without waiting for the adjudication of the I-751. By law, your I-751 needs to be adjudicated before the N-400 can be adjudicated. Thus is a way, an N-400 application forces the adjudication of one's pending I-751.