This site
https://www.josephsmithpapers.org/person/alexander-mackenzie#13298169327192813834says that Ranald's father, Alexander Slidell, "changed surname from Slidell to Mackenzie, 1837, to honor wishes of maternal uncle."
and cites this source:
“Mackenzie, Alexander Slidell,” Dictionary of American Biography, 6:90; “Mackenzie, Alexander Slidell,” in National Cyclopaedia of American Biography, 4:527
maybe the source names the uncle?
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and you will need to do further research to rule this man in or out. I'm only mentioning him because he died about the same year that Alexander changed his name from Slidell to Mackenzie. I've seen both 1837 and 1838 given.
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Alexander Kenneth Mackenzie
d Parramatta, New South Wales, Australia 26 Dec 1838
buried 28 Dec 1838 St John's C of E Cemetery
[burial records at ancestry.com]
https://www.familysearch.org/ark:/61903/1:1:Q2WL-2YZ1