Will 7.5" axle fit on 8.8"?

I'm assuming you're talking about the axle shafts inside the 8.8" axle. I don't think one from a 7.5" will work because i think they're smaller and have different size splines on the end. Both axle shafts from the 8.8" axle should be identical because they're no reason to manufacture them differently so it SHOULD fit in your car with no problem. Take the old one out and compare it to the new one to make sure and count have many splines are at the end of the axle shafts. I think they made a 28 spline and 31 spline axle shafts for the 8.8" but i don't know specifics about that. If the count is the same you can garauntee that it'll fit (assuming it's out of a mustang).