round2it,
Conventional wisdom in auto/truck design is to make the frame or unibody as stiff as possible. Let all the deflection be in the suspension as it is designed to work. Flexing frames will alter suspension geometry and can create undesireable handling characteristics, squeaks and groans. In severe overloading the frame can be pushed into permanent yield, meaning it will not return to the original shape. Amazing what happens out there in the "field". Add in years of corrosion and the body/frame loses some stiffness to this as well. Auto frames/bodies fail in a ductile manner, unlike a cement building that will crack catastophically in a brittle failure mode. Mounting buildings on some sort of flexible mounts or providing some way for the building to move between floors makes a lot of sense. Avoid the overload and brittle failure of the cement/glass. (I left Mexico city a couple of days before their biq quake in 1985, The highrise I was in, you could feel it wiggling around on a windy day. Weird).
Note you have seen the ads where a car is crashed and the energy is disapated in crumble zones in the frame of unibody. In that case, the crumple zones are designed to deflect in a high load crash, but not flex in operation. Like your thinking, but stiffness is your friend in a car/truck. Sorry for a basic response, not my area of expertise. Happy miles.
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Ramble on | 
