Since you are aware of a bully who is harassing his fellow employees, you should definitely go over your boss's head, but the more basic question is whether any members of management are using their heads in the first place. After all, some of the employees have already complained to management, all to no avail.
There are tremendous costs associated with this type of harassment, not only in terms of potential legal exposure, but also in terms of decreased productivity, increased turnover, and loss of goodwill, to name just a few. When informed about this type of problem, most rational managers take immediate steps to stop it and take formal steps to prevent it from recurring in the future.
When management turns its back in these types of situations, it is a clear sign that leadership is either poorly informed or totally uninterested in the needs of the employees. In either case, this type of mismanagement can easily drive the trucking company right into the ground.
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