Just watch the box on your truck as it moves up and down left to right as the frame flexes. If it did not flex it would break from fatigue. The cab leans one way and the box the other. This flex happens every time you enter your drive if is it is above the elevation of the road or a parking lot or an off camber turn. The steel of the frame is designed to do this along with the cross members to allow a bit of give. this is why there are rubber pucks at the cab and core support mount points to absorb some of the flex and prevent sheet metal stress damages.

Slick frames are a riveted ladder construction and if the front left is raised by the suspension the right rear will dip with the right front and left rear remaining the same. It is one of the reasons the uni failed as under load the frame flexed and the body did not causing the doors to open or jamb shut.

The Crown Vic is a unitized construction vehicle. The mounting points are rigid and unmoving with engineered forged steel inner steel mount points that are encased within the sheet metal structure and held in place with resin glues. The CV cross member is cast aluminum and is not designed to move with a frame. it is engineered to be a rigid mount point for suspension attachment. If you really look at one when it is mated to the car on the line as a component with the engine in place on the mounts and all the suspension attached. It was made for ease of production and install not a cool piece to be used for suspension on a truck with twice the front weight bias of the car it was intended for.

And lets get in to weight. Yes the CV and a truck weigh similar on the scale but that means jack. The CV has a 50/50 weight bias front and rear. A truck has a 70/30 weight bias meaning that most of the weight under normal driving is on the front wheels. This load increases exponentially as brakes are applied as more bias is shifted to the front.

Remember this is a casting not a forged piece. It has potentially the same metallurgical strength as the pot on your stove.

But then again i may be wrong.

Garbz