Grigg,
The original diagrams in the some of the literature and manuals shows lines connecting the cab based on these mounting points. The front crossmember/radiator support carries the entire front end from the firewall forward. The rear shackle worked the same way so the linkage looks like a diamond shape from above. Then they went to the paired shackles which took away the rear diamond. This is the GM explanation so it must have been what they were seeing in claims back to the dealer that prompted it.
The forward chassis rail I would interpret as the outside of the frame rail. The shackle location there helped so the cab was not bouncing on rubber bumpers but the shackle movement itself. The movement of the cab against the frame must have transferred the stress back into the cab structure instead of the shackle.
https://www.flickr.com/photos/advance-design_parts_co/8349389839/in/album-72157632440242503/
