I am nearly certain that I am a descendant of both Griffith and David Llewellyn via John and Elizabeth (Llewellyn) Young. A record of the descendants of William John of Gwynedd (by the Gwynedd Friends Meeting historian, with citations from wills) shows that his daughter Margaret Jones (she may also be called Margaret John) married (1) Rowland Ellis and (2) David Llewellyn. He has their daughter Anne Llewllyn marrying William Young, and their son John (who was a captain in the Philadelphia militia during the Revolution) marrying his second cousin Elizabeth Llewellyn. That seems pretty clear to me. A few years ago distant cousin in Texas sent me a similar genealogy. Elizabeth Llewellyn's dad was John Llewellyn, who was a physician, and his dad was Griffith. I also have Griffith's wife as Elizabeth Thomas (which I believe), though another source listed his wife as Margaret Lawrence. Hope that may provide a little confirmation or further leads!