Car carrier/pumpkin and 1/2 ton truck pickup units are not the same. The shapes of the pumpkin are significantly different and do not interchange between main axle housings.
That's not to say you can't mount a complete car rear axle in the same spot as the original truck axle, but that's going to leave you with 5-lug axles/wheels in the rear, not to mention a weaker rear axle overall.
