So you want to file a grievance for your manager trying to make his operation better on his reports? For what? Are you not getting paid your guarantee of 3.5? If you aren't (for some odd reason) then you have a case. If you are getting paid your 3.5, you have nothing. You can't grieve operation decisions a manager has the authority to make when it has nothing to do with the union work. He's getting union people to move boxes to union drivers to drive to a different set of union workers.
You don't have a case. Any attempt to make it one is a stretch.
Now, before he started to move this volume.....was everyone always getting overtime? To the point where it seems as if you needed another employee working? If that's the case, and you can prove he's diverting this volume to avoid hiring another hourly, then you would have a case there too. But if there was no significant amount of OT before he started diverting, then you still don't have a case. Grieve something more important if that grievance form is burning a hole in your pocket.