i have also seen that warning on semis also just didn't come to mind but with the whole weldint sub frames and all that in is a good point on my '70 Chevy i had to weld in some 1/4" cold rolled plate steel to fix my frame cause it was cracked and rusted badly above the rear coils
Nate

Link to my photobucket for pics of my vehicles
http://s238.photobucket.com/albums/ff17/OldIronChevy/