I was injured at work, post office, by a runaway cage of mail that was shoved thru door by truck driver, contracted by post office

The truck driver shoved mail cage through dock door, without control, and it caught me on my right side while I was bent over another cage of mail, unloading it. While, I did not fall over, I caught myself with my left hand. I have been dealing with workman's comp since January. The driver was let go that day and the trucking company is contracted by the post office to deliver the mail. I now have a pinched nerve in my lower back and my left hand has carpal tunnel. I have pain almost all the time in my back and right hip and left hand and arm. Can the trucking company be held liable for my injuries, even though work has been covering my medical expenses?