I think I have finally found Nimrod Donaldson in 1820. I think he was living, as a farmhand or boarder, with his future in laws in Greenville, SC. William McCullough, Nimrod’s future father in law, had an extra male age 16-25 in household in the 1820 census. Nimrod was born in 1803, thus making him 17 in the 1820 census.
William McCullough’s only child, Sarah, turned 16 in March 1824. She married Nimrod on November 23, 1824. Nimrod stayed close to the McCullough family until William McCullough and his wife, Jane, died in the 1850’s. Nimrod Donaldson was the only child of his parents to stay in South Carolina after his siblings either died or moved out of state. The McCullough’s were very wealthy farmers so Nimrod probably had great motivation to stay close to them. I also think that Nimrod probably didn’t want to move his wife away from her parents because she was an only child.
I am glad this mystery is solved, hopefully!