Can an employer force an employee to work out of state temporarily?

Obviously they can't physically force you. However they can request that you move and if you don't it could cost you your job. So unless you have an employment contract or union agreement that prohibits such a move, or a temporary re-location is against the company's own policy, or your move as to do with some form of workplace discrimination, your employer's action is perfectly permissible.

The fact is that in an employment relationship, an at-will employer can increase/decrease salary/hours, promote/demoted and generally impose whatever terms and conditions of employment that it deems necessary. This includes employee relocation. For your part, as an at-will employee you can choose to continue to work for your employer or not.