Hi, Lonnie,
Just so you know, there were Diffenderfers not only in Pennsylvania but a large family in Virginia/West Virginia by this time. I checked a database of more than 4,000 Diffs and found no Plunk or Plonk. That said, please contact me directly at Pamalot at nc dot rr dot com and I'll try to help you find more clues. It all depends on where you already have searched.