You didn't happen to weigh your 9" assy vs the 8.8" did you? Based on some other numbers I've seen here, I'm thinking something like a 9" w alum carrier/pinion carrier, alum spool, alum yoke, lightened ring gear, and gun drilled axles, along with drag brakes ends up being a significant weight loss.
Yep my 9 inch from Currie, when I first got it, was in fact lighter by 5 lbs, but I attributed that to the spool and the aluminum pinion carrier (not the center section) vs the 8.8 which had a Detroit Locker.
I was looking at first of how you got the 30 tall to work but then I noticed its 9 wide so I am thinking that is the difference. I had to do some mods/trimming on the inside when I first ran my 29.5X10.5W.