From what I can tell reading online, medicare and social security should not be taxed from this 1099-DIV money
That is correct.
I am trying to determine how much back taxes are due to this income specifically due to the dividends.
Perhaps the best way to do this is to figure your taxes two ways:
1) Without the dividend income.
2) With the dividend income.
The difference between #1 and #2 is the tax due to the dividend income.
Just to stir the pot a little...
...you could also figure your taxes with the dividend income and your payroll income, and
1) Without your wife's temp job income
2) With your wife's temp job income
...and you could also figure your taxes with the dividend income and your wife's temp job income, and
1) Without your payroll income
2) With your payroll income
Good luck!