Moving the axle back is not a huge deal. Any truck repair shop, truck equipment shop (one that sells/installs decks, boxes, etc) or frame shop should be able to do it. Basically disconnect all hoses and wires from axle/suspension, slide it back to where you want it, drill new holes, bolt it down, extend hoses/wires/driveline and reconnect.
As far as 19.5inch wheels, only the dealer could tell you as it all depends on how the truck was speced when it was built.
2007 Haulmark 3329DS, Freightliner Coronado Chassis, 515hp Detroit, Meritor Freedomline 12speed Autoshift, 10kw Generator, In-motion Satellite.