"Durnford", in Wiltshire, just north of Salisbury, is "Greater Durnford" and "Little Durnford". This is on the Avon. "Durn" means "hidden", thus the "hidden ford". There is a great Norman church still there, as well as the older cairns, etc.
The names Dunford and Durnford abound in the area phone books.
Another "version", and probably true, is that some Irish immigrants to the U.S. had a very long and unpronouncable name, so the Immigration Authorities just shortened it to Dunford.
So, take your choice! The oldest version, however, is the Salisbury version.