I'm also search for Va. Diffenderfers-- 1800s and 1900s.
Oldest first. About a year ago I came across on the web, and downloaded, what I think is a Va. State Archives index card with the name Frederick Diefenderfer on it and a date around 1804. I didn't pursue it, but have you called Va. State Archives? I understand they're super. National Archives in DC said they sometimes have better files of the period. Let me know what you find.
Also, I am looking for any descendants of James L. Diffenderfer (1838-1912) who married Viola Hill in SW Va and had a number of children--Leander, Mattie, etc., and I believe a grandson Bruce Leander Diffenderfer. I have oodles of info on James! You can contact me directly at LWA101@aol.com