From what I understand the last name was originally Rathkey and somewhere around 1795 one of the sons was purchasing property in Blair County, PA. The name was recorded as Rodkey and it has been that ever since.
I do know that there were 6 Rodkey's that fought during the Civil War, many of them being from Blair County, PA. 2 fought at Gettysburg, Robert and James, both survived and James is listed on the Pennsylvania monument, not sure why Robert isn't listed.