I don't know the American market, but here where I live, this is true indeed. Of course, you have to be THAT good.
On a side note:
As long as you have a boss and/or you work for a company you need always keep in mind that they have their interest which contradict your own and they will always pursue their interests first. The trick is to make them believe it's in their interest to give you what you want.
The difficult part is getting to that point.

Sounds like you have a sort of mentor looking out for you. In her position she is likely privy to the nuts and bolts of the business, and also what you can do to improve your chances. Make good use of the advice she offers - we can tell you from afar what to do and what to don't, but none of us know how your company runs.

If they cut back in response to Obamacare, I think the problem here is internal politics. You're probably not playing the game as well as the other people are.

I'd also point out that this isn't a well run company if they're cutting back on hours due to that. I'd also suggest that getting a new job at a different company is likely to be a better long term solution as it sounds like this company is CTD.

Unless you're a member of a protected class AND you can prove that this was the cause of the lost promotions, the likelihood of winning a lawsuit is pretty much zilch.

IANAL, but that's what an attorney would likely tell you. Obviously, there may be more information that points towards discrimination, but the SCrOTUmS bend over backwards for employers, so I wouldn't expect to win a suit unless it was completely flawless.
It depends a great deal on where you are and how in demand your skills are. Generally, you don't get pay raises unless they're baked into the contract. I think the only times I've gotten raises in the US was the result of a union contract calling for it. Even just getting boni is rather unusual these days.

But, if you've got experience and are in a demand field, sometimes threatening to quit is what needs to happen. The key though is that you have to be in a position to follow through on it and preferably already have job applications out there and HR checking references. If they don't see evidence that you're really going to do it, you're much less likely to succeed.

Anyways, around here you can probably get away with that, our unemployment rate is back to pre-recession levels, but in many places the employment market is still too soft for hardball.

What do you do? Also, change employers?

Without knowing all of the details I would say this specific reason is highly unlikely. A policy of not promoting based upon comparative compensation levels is counter productive for an organization when measured in risk/reward terms. Any form of discriminatory human resource policy exposes the enterprise to the risk of litigation. The cost of defending such litigation is potentially enormous, particularly if such a case actually goes to trial. And trust me, very few make it to trial because the business considers it less expensive to settle out of court even if the settlement runs five figures.

So, and I don't mean to make this sound condescending in any way, whatever issue they have with the pay differential between yourself and shift managers is likely peanuts compared to the potential cost of actually engaging in a discriminatory policy of some sort.

thanks for the answers. Will keep all the information in mind.