Back axles have been musical chairs through the years, but the point was that the bigger truck got a more robust setup than the 8.5"/8.625" half ton rear with sloppy spiders as the miles march on. The models I am familiar with were 8.25" (half) or 9.25" (HD) front.
I'm of the mindset that if your occasional use, special duty pickup gets 14 mpg, it's not that different than it getting 12.
I propose that the torsion bar setup doesn't ride poorly unless it's been cranked and the jounce bumpers are trashed...well, unless the rest of the system (pitman, idler, tie rod ends, intermediate shaft, column bearing, and steering gear) has been neglected.