What if federal tax withholdings on w2 does not match withholdings on paystubs. In fact, none match. Paystubs show higher tax withholdings than W2

Then you go with the W-2, most likely.  Some things typically don't go through pay stubs, like compensation and tax withholding due to a vesting of an RSU or the exercise of a NQSO or some other action associated with an employer stock incentive program.  Anything like that happen in 2018?

But it doesn't hurt to check so I'd advise calling the payroll department and asking for an explanation.

Tom Young