You don't say, but can I assume both 48's are Chevys.
If so, the 3/4 ton is a longer wheel base than your 1/2-ton, so one of the issues you would face is determining what to do to lengthen the torque tube.
GMC did make long-wheel-base 1/2-ton trucks. I have no idea how they handled the longer length underneath, since the only one I've ever crawled under had a V-8, automatic, and 70's 4-wheel-dirve.
To swap rear axles you would either need to swap transmissions too, or convert the 3/4-ton transmission to connect to the torque tube.
I think if it were me, I'd either look for the later 3/4-ton differential, with better gears, or swap to a later axle assembly using an open driveline.
