J-series trucks did come with the heavy 6x5.5" C20 rear from 1980-early 1986, but only the J-10. I had one in an 82 J-10 Laredo I parted out some years ago.
http://www.ifsja.org/tech/figures/db.html#rear
A rather annoying axle to try and find due to the limited years the heavier C20 was used and lower production of J-series and WT models during those same years. If you get one, you should get two of them, since you'll be screwed if you break a shaft. The nice thing is that if you can find them, they are usually free-$50 since people never know what to do with them and typically consider them useless. (Good for those of us that know what to do with them. :mrgreen: )
