Check the mounting bushings under the cab. They wear out / dry rot and get smashed under the weight of the cab. You can tell if it is , they will have big cracks running up and down the sides of the rubber or sometimes parts of the rubber is gone completely. When they wear out one side of the cab will lean down lower the the bed. The bed is sitting flat on the frame so it can't move up or down.
You must have gone to a professional welder and not a professional body shop. A body shop guy should have noticed that and pointed it out to you.