I can't really remember how to tell, but it seems like the 8.8" is bigger than the 7.5".......maybe I'm wrong, it's been awhile since I've been cursed with a Ford. All 4.0 V6 and V8 powered trucks and SUVs came with the 8.8" if that helps.
I ran the 7.5" in my Ranger pre-runner. Blew the cluster gears out twice hitting second gear, twisted a shaft too(with 31" tires!). Yeah, avoid the 7.5". It's a punk compared to even the D35.