Wether you have a route or not, it looks like you are near the bottom of the seniority list. If there are no other routes available, and the 2 guys below you are sent home, it looks like you are next on the list! The unassigned drivers still have the right to bump the least seniority drivers if that is the only work available. As long as no other driver is working that has less seniority than you, I don't think it is something you can grieve. They actually are following the contract. That is how I see it with the information you have given.
p.s. No I'm not a steward, but I did stay at a Holiday Inn Express last night!