It seems William wasn't sure what state he was born in. It could be he moved to Maryland when he was very, very young and really didn't know.
The 1850 census under the surname Jeffries has William 26 born in Maryland, but it's not unusual for the enumerator to default to the state of residence as to where the person was born. The 1860 census under the surname Jeffers has him 31 born in South Carolina. The 1870 census under the surname of Jefferies has him 49 born in North Carolina. The 1880 census is the most interesting of all. Under the surname Jefries it has him 57 born in Virginia and his parents in Texas.
His children were not much better at knowing where he was born. His son William believed his father was born in Virginia (1900 and 1920 censuses). His son Herman indicated his father was born in Virginia (1910 census) or North Carolina (1900 and 1920 censuses).
Any Jeffers who lived in Cecil County around the 1830's to the 1850's may be related. At least it's worth checking into. Have you looked at estate records for Cecil County?
I suggest trying to determine the exact date of his birth. His tombstone may have this info. The church he was a member of may have something in their records to help you. Since he was buried in the Methodist Episcopal Cemetery it's a good place to start. You might try one of the local historical societies to see if they can help you. I believe Cecil County has one and Elkton may have one.
BTW born in 1829 and married 12 years later in 1841 seems a bit young to get married even back then. More likely he was born ca 1823.