You really didn't get an answer to your direct question of "do I need to make a payment to the IRS now?"
The answer to that question is "probably not, but it's really impossible to say without knowing your exact situation."
What can happen with you get an unexpected windfall where taxes weren't withheld - this is your situation - is that you might end up being "underpaid", (a bit of tax jargon with some specific meaning), and being assessed underpayment penalties when you file your income tax return in April, 2020.
Most taxpayers will avoid being underpaid if they:
1)owe less than $1,000 in tax after subtracting their taxes WITHHELD and available tax credits,
OR
2)if they paid at least the lesser of
a)90% of the tax for the current year, or
b)100% of the tax shown on the return for the prior year. (If last year's return shows AGI over $150K (for married filing jointly) then change that "100%" figure to "110%.)
If the amount of gain on your sale of stock is relatively small and if in 2019 you'll be working as a W-2 employee with your withholding amounts pretty much unchanged from the amounts withheld in 2018, then it's highly unlikely that you'd be assessed an underpayment penalty. But this is one of those situations where nobody here can give you a direct "Yes" or "No" to your direct question of "do I need to make a payment to the IRS now?" because nobody here has access to your personal income tax situation.
Tom Young
