Roberta,
Hi, as others have indicated the more known information the better.
As you probably know, when you get this far back it is very much a jigsaw puzzle.
If I have it correct, in the 1851 census you have
John Wilkie, 41
Elizabeth Wilkie, wife
James, 12
Margaret, 9
Agnes, 9
Jane, 7
Grace, 5
Catherine, 3
It is slightly complicated by the two girls who appear to be twins, but by naming tradition we should probably have his father and mother as James and Agnes, and her mother as Margaret. This gives us Campbell's to look at and look for further family names, which might give pointers to his family names.
Is there any more information on the marriage you have?
I notice that in 1871, he is widower? He seems to have lost a couple of years. His real age? Or doesn't want to be old!
There are three possible John's in Fife in 1809 James and Isabel Jackson in Newburgh look interesting as they also have a daughter Grace.
It is worth noting that Wilkie is not that common a name in Aberdeenshire, particularly John. When searching on SP there is only one page of John Wilkie's in Aberdeenshire. There are many in Fife and Angus.
You may well already have done this, but it is very much worth building speculative trees, both for possibles John's and for any Aberdeenshire matches that make you think he or his family are from Aberdeenshire. I have on several occasions found people this way, particularly by building DNA match trees.
More than happy to try and help further.