'96 Explorer V-6 OHV, CEL and codes P0171 and P0174 System lean both banks

I have a '96 Explorer (OHV) 107,000 miles that runs/idles flawlessly. A few months ago I got a CEL light. Had Autozone read the codes and p0171 and p0174 are present. I replaced both oxygen sensors (before cat. converter), repaired an exhaust leak in the lower EGR tube (new tube and EGR valve) as well as the DPFE sensor. I am told that this could be a vacuum leak. Propane enrichment does not indicate a vacuum leak. What is the most likley cause of the problem ?