Class 8 chassis with aerial devices: Taxable or nontaxable?

We are a truck equipment distributorship that sells vehicles outfitted with aerial lifting devices. The chassis are rated 33,000 pounds gross vehicle weight or more. In order to install aerial devices onto the chassis, we first must make substantial modifications to the chassis. The aerial devices are then bolted to the chassis. Is it true that our retail sales of this vehicle may be conducted tax-free?