The people who are unhappy are the ones who want to work preload then drive, but can't because higher seniority drivers are working preload, right? And these unhappy drivers want to force the drivers who are working preload to drive afterwards so they will stop working preload, allowing the unhappy drivers to do both?
I would say that seniority rules, the higher seniority drivers have the right to work the preload and go home after if there are other drivers who would be sitting at home otherwise. The unhappy drivers should try to request any overtime that the lower seniority drivers would be getting while covering the preload workers' routes. Everyone would get to work as much as they want according to seniority, win-win.