The original sport utilities were simply pick-up trucks with an aftermarket camper shell bolted to the top of the truck bed, and which utilized a rear-facing, bolt-in truck bed seat. Modern SUVs are far more complicated and integrated than the conversions of decades ago, and may be subject to laws that their forebears were not. However, you should consider the civil lawsuit repercussions of modifying your truck's factory safety systems before selling it.
Because almost all SUVs started out as pick-up trucks, conversion won't change the truck's vehicle weight class or emissions status. This alone means that under no circumstances is SUV conversion totally illegal; such conversions are considered aesthetic enhancements as long as they don't require modifications to the frame.
Modifying your truck's frame isn't illegal, but it may not be smart if you ever plan to sell the truck. Selling a vehicle modified in this way negates any factory liability for its crash-worthiness, so you assume all risk if the buyer ever crashes it. Under these circumstances, even an air-tight legal waiver may not be enough to protect your assets.
It is illegal in many states to tamper with or remove a vehicle's airbags, be they frontal impact or side-curtain. This shouldn't be a problem for standard-cab pick-up trucks that only use frontal-impact airbags, but extended cab trucks with rear side-curtain airbags may be a different matter. Conversion may be a problem if it entails removing, relocating or rendering inoperable any of the vehicle's stock airbags. Consult you local and state laws for compliance.
Laws regarding seat belts are similar to those regarding air bags, but may or may not be enforceable in your state. Many states (like Florida) don't require that rear seat passengers wear a safety belt, so whether or not your new SUV's rear seat belt anchors meet federal crash standards may be a moot point. This should be a non-issue if you're replacing the entire truck cab and bed with an SUV body, but check your local seat belt laws for compliance.
You should re-title your truck as an SUV after modification, even if it's not required by your local jurisdiction. Re-titling is a wise move after any major visual modification; the truck's frame-mounted VIN number will verify your registration, but you may have trouble finding it in the dark by the blue light of a police car.
