A question of pay discrimination?

I work as a custodian in a good size (600 people more/less) bargaining union. I am actually considered a floater, I float around and fill in positions that are vacant or where people are off. All the floaters are grouped in one department, the Utility Workers (UW) department. There are 30 utility workers. Utility workers are not guaranteed hours, if there are no vacancies then there is no work for UW. UW are placed in pay grade 01, they do not move out of that pay grade till they bid on a permanent position. Now for my questions that I need a second opinion on: The union contract states that UW are not eligible for shift differential. The contract also states that anyone in pay grade 01 (which are just UW) are not eligible to be plus rated. Can they discriminate against one department like this?

It's not discrimination. It is perfectly legal.
Discrimination only occurs if a member or members of a protected class are singled out. A protected class is race, gender, religion. They can do whatever they want with your pay grade.

The Union signed the contract (ratified) now you have to live with it till the term ends.

Union contracts are for the high seniority members first. Lesson learned.

This is not discrimination, but rather definition. There might be an argument if you could demonstrate that pay grade 01 is invariable filled with members of a protected class or filled in such a way that if the members were discriminated against, it would on the face of it seem as if it were on the basis of race, sex, national origin, religion or what have you else that is prohibited.

But otherwise? You would need a very creative lawyer.