As you probably know, there are many, many James Alexanders who were born in the 18th cent. in Pa. and in Va. The one who is a son of AA (Sr.) and Margaret Jane McClure was b. in Augusta Cty., Va., ca. 1766. He married Martha Tedford (Telford) in Rockbridge Cty, Va. in 1794. They moved to Ky. and, later to Lincoln Cty., Mo. where James died in 1849. Although some of the information on Ancestry.com is useful, some posters simply aren't careful about the data they post. There's no substitute for primary source data.
you might check the spelling of his wife's name...it might be "Reid". Several of my line of Alexanders married into the Reid family of Augusta/Rockbridge Cty., Va.