A SRW pickup and a cab/chassis use the same width rear. The spring perches on a pickup rear are mounted wider than a cab chassis. A pickup Dually rear is approx. 4" wider overall than a SRW or cab/Chassis rear. The frame of a Cab/chassis is 34", the frame of a SRW and DRW pickup is 37.5"
Not sure what the rearends actually measure off the top of my head. I would think if you have one thats 45" at the perches it would be from a pick up and 38" would be a Cab/Chassis.