Hi Kimberly!
Meanwhile I found out a little (unfortunately only a little) more about the name.
The most former mention of the name I found is in a book from the middle of the 18th century: "Schicksal der Wigandsthal - Meffersdorfischen Kirchfahrt" from Johann Ehrenfried Fritsche. It tells the story of the inhabitants of two complete villages ("Reichhennersdorf" and "Zieder") near Landshut in Bavaria, who had to leave their home and emigrate to Silesia because of their protestant religion. That happened in 1687, and "Effenberg" is one of the mentioned names.
So, in 1687 there seem to have been Effenbergs in Bavaria, near the town Landshut.
There is an old word "Effe" (Hildegard von Bingen called it "iffa" in 1150) for the elm tree (german "Rüster" or "Ulme").
This could be the origin of the name "Effenberg": someone who lives on a hill with many elm trees.
But "effe" seems AFAIK to be only known in Rheinhessen (on the right side of the Rhine River, between Mainz, Alzey, Bingen and Worms). On the other hand, there is AFAIK no concentration of Effenbergs in Rheinhessen.
"Effenberg" is still a mystery. ;-)
Best regards
Harald
http://www.effenberg.de/