Milburn is still a fairly common name in the greater Hexham area although, historically, the Milburns were based further north along the North Tyne valley nearer the border with Scotland. It would be worth contacting Hexham’s Old Gaol Museum which has a collection of documents relating to the reivers https://museumsnorthumberland.org.uk/hexham-old-gaol and the Bellingham Heritage Centre who are much closer to your ancestral lands and have an impressive archive of their own https://bellingham-heritage.org.uk/contact.html