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Dually's have the same width rear axle as a pickup. The cab and chassis however have a narrower rear housing. Backing plates are closer to the spring perch.
I found this place by Googling them, but I can't tell you how repitable they are....http://www.duallyfendersusa.com/Here is a grainy pic from their site, of what they have to offer(scroll down their main page till you see the same pic if you prefer):Here is their pricing and info page:http://www.duallyfendersusa.com/fenderprice.htmIf you use Factory Flares, you can use your current bed(if usable), or Standard LWB/SWB Aftermarket Bed Replacement panels.... You simply mark and mount the flares to the exterior of the panels, then from inside the flares, mark the excess standard panel to be trimmed.... The excess material needs to be removed, to allow the dual rear wheels, or extra wide wheels(It was popular from local Truck Guys, to replace the dual wheels with some widened single rims with Mickey Thompson Pro Streets) proper suspension travel....
Do a search on the forum. I posted a site that has dually fenders and they have a great reputation.