The part that loses me is the "third member". I didn't know GM ever made a 3rd member rear.
Though, looking in my 77 GM Light Duty service manual, they do show a 3rd member type. It has a 12 1/4" ring gear. BUT, it says it has an 11,000 lb. capacity. I know the 14 bolt is rated at 7,000 lbs. I've seen the 12 1/4" in the manual before, but figured it was for step vans, and possibly C/60 models and motorhomes. I sure haven't seen it in a pick up. In fact, I've never seen one, so you will have a really hard time finding one.
Maybe in Hemmings Motor News you'll have more luck? I doubt you'll have any luck in this forum with an axle like that.
You might want to look in classifieds that cater to larger trucks and motorhomes. Maybe it's in C/50's C/60's C/70's and motorhomes. Oh, and it's also a single speed differential. So if you see "split rear" in an ad, it's not what you want.