If I am a salaried worker on an 84 hour/2 week week pay period but only get paid for 80 hours and I work more than 90 hours, is this legal?

If you are salaried, you are not paid for the number of hours you work: you are paid a weekly salary equal to 1/52 of your annual salary. For salaried employees, it does not matter (except as below) how many hours you work--you are paid the same.

Note that some salaried workers may be eligible for overtime--being paid on a salary basis does not, by itself, make you ineligible for overtime under the Fair Labor Standards Act (FLSA) and regulations implementing it. To not get overtime--to be exempt, as the law calls it--first, you must be paid a salary (not hourly wages); then your job must also meet one or more of the tests for exemption, which can be found on the Department of Labor's website under "wages," then under "overtime." The main tests are the executive (which really should be called the "managerial" test, since it applies to non-executive managers), administrative, and professional tests. Compare those tests to your job. If you don't qualify for an exemption, it may be that when you work more than 40 hours in a week, you are entitled to additional pay. It can be difficult to calculate this extra pay for a salaried worker; if you think you are entitled to it, discuss the matter--and how much you may be entitled to--with an employment attorney.