Then if you know how to unload you can't argue that you're not trained. So you can only argue seniority. Make sure that you pay attention to which loaders stay and load when they send you out. If they are junior, and know how to unload, you can grieve. You would say, "On July 1, 2015, supervisor Bob sent me to Unload while junior employees Jim and Dan remained in PD1. Jim and Dan are trained in unload. Respect my seniority! Junior employees must be sent out of the area first."
This depends on your building. You can't demand to do a specific job everywhere. Maybe not even anywhere, technically. You can't just tell your center manager you want to work in small sort and he needs to kick out a lower seniority employee to accomodate you.