I believe the shift differential is a legal requirement that they cannot change, unless by law. They may be in violation with state laws. Check with an attorney and also check your state laws regarding shift differential compensation that may include violation of excluding overtime pay in the shift differential.
A visitor of this site offers this as a possible solution...
As far as two workers receiving the same pay, even though one employee on one shift is doing more than the employee on another shift...that's something you can't change.
What you CAN do is to someday...have a talk with your immediate supervisor (the one who regulates your pay increases) and point out how valuable you have become because you're doing "this, this and that" compared to the job responsibilities of the daytime crew. Show how productive the department has become or find something that you can point to that has improved due to your handling of the situation.
You can politely ask: "What expectations can I reasonably hold for a raise in the near future for my contributions?"
If raises are being given, this may give your manager something to think and act upon, to your benefit. If raises are not being given (this is the norm currently), and you're doing the job of two people, welcome to the working world as it is now. That's becoming so commonplace now, I'm afraid.
But...you might be able to squeeze a small raise for yourself and this is at least something. Might pay for the gas to and from, huh?