I am deploying in about 6 months and my civilian job is holding back a promotion until I come back. What should I do?

On the regulatory side, the Uniformed Services Employment and Reemployment Rights Act (USERRA) states:
[Service members] "shall not be denied initial employment, reemployment, retention in employment, promotion, or any benefit of employment by an employer on the basis of that membership, application for membership, performance of service, application for service, or obligation."
Violations can include:
"if the person's membership ... or obligation for service in the uniformed services is a motivating factor in the employer's action, unless the employer can prove that the action would have been taken in the absence of such membership ... or obligation for service"
I would follow CPT Forwood's advice and touch base with an ESGR rep. Without knowing the details of the promotion decision by the employer, I could not say that it is a violation of USERRA, but the ESGR would likely contact them for details.
My advice would be to also sit down with your boss and have a real conversation about USERRA so you are both on the same page. The worst outcome is to strain the relationship between you and your boss due to a misunderstanding.

Recommend touching base with your local ESGR rep. They’re known for highlighting great employers who are supportive of their Guard and Reserve members. Sometimes a preemptive pat on the back helps to motivate managers to do the right thing. If not, ESGR should also know the right stick to wield.

Pray the put it in writing and then ride that lawsuit into retirement.