Damn Chevy frames ..... I completely boxed in the whole back of my frame and did frame plates all the way up front . You don’t think the urethane Would be any better?cab is moving because of the frame flex. If you remove the rubber body mounts and replace with solid now the cab floor will flex and crack to shit. Leave it as is until you cage it. then solid mount everything.
It has it done it in the past before I did my bed cage and all my frame plating nothing after that .is your bed hitting the back of your cab near the rear window?? if so thats frame flex caused from the shock mount leverage on your bed cage. the only way to counter it is a full cage unfortunately, frame plating never works. i had a buddy put solid ones on a truck with no cage and they ripped the sheet metal and punched through the floor. when i caged my truck i tied the cage to the cab as most do and ran stock body mounts and it NEVER moved. i had 4 tubes comin out the back and two through the front and there was never any cracks.