Oh man does this look like all kinds of fun, based on what you've posted your truck started life as a 1 ton C30 2wd and was "bubba'd" into a dually (Bubba'd lol i think i gonna steal this) by way of what appears to be a NP205 part time transfer case a Dana 44 front axle from a full time 4x4 (np203) single rear wheel truck and the original Corporate 14 bolt full floater rear axle appears to have been widened to the width of a Dana 70 DRW pickup truck axle. If you want to pull the dually flares you can but you will have massive cutout wheel wells as if you were trying to fit massive tires without lift not the small square wheel wells of the dually flares or the front end, if your going to go back to SRW than by all means i would find a replacement SRW axle (I would replace that axle just because someone widened it and i don't know who did it or how well and wouldn't be able to trust it) with the gear ratio matched to your front axle unless you find a matched set (front and rear axles from the same truck) for a good price. I would also recommend you look into getting a set of manual locking front hubs you will get slightly better MPG and have less wear and tear on internal front axle components, however to turn on 4x4 you would need to get out and turn the hubs on as well as shift the transfer case.
I never realized that Oshawa trucks got sold in the US, I always figured that with all the plants in the US the Oshawa plant just built trucks for Canada, learn something new everyday.
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