they wont fit a 3/4 ton chevy...the brake caliper/rotors are too big. My aftermarket 16" rims clear the caliper with about 1/2". why would you want to go to a smaller rim? Im not sure about weight carrying capacity of the 15" rims either.
you maybe/should be able to fit them with wheel spacers....just throwing that out there never tried with a dmax so really have no idea but, my bogger truck needed 1/4" spacers to fit 15's, you can also grind the callipers a bit too
I only have about 1/2" clearance with my rims...I was worried when I was getting ready to put them on. I wouldnt grind anything on the brake system...if you got in an accident and they saw that...bad day for you.