Actually, the McCulloch and Gray families are not part of my family lines because I descend from William Hord, brother of Thomas [Eldridge] Hord. Thomas [Eldridge] Hord was one of my 4xG Uncles. So, I have not researched McCulloch and Gray lines.
However, it appears that Miss Mary Elizabeth McCulloch was the daughter of Benjamin McCulloch, b. 1789 Lunenburg, County, Virginia and a Sarah who may be Sarah Caswell nee Lytle and that this McCulloch line goes back to Alexander McCulloch of Ireland. The Rev. Arnold Harris Hord in his book does not name the mother of Miss Mary Elizabeth McCulloch although he names Benjamin McCulloch as her father. On page 112 of that book, he writes that Benjamin McCulloch Hord was a cousin of Confederate General Benjamin McCulloch (killed at the battle of Pea Ridge, Arkansas).
The Benjamin McCulloch who was father of Miss Mary Elizabeth McCulloch was born 16 Apr 1789 and died 10 Aug 1847 at McMinnville, Warren County, Tennessee. He was a grandson of the Alexander McCulloch b. 1715 in Ireland.
With regard to the birthplace of Thomas [Eldridge] Hord it may have been Claiborne County because his father moved to Claiborne from Hawkins at some unknown date in 1802 (a search of land records might provide a clue to the date) while Thomas was born on 31 Aug 1802. The father, William, returned to Hawkins County in 1813.