As far as I know, if the input flange is the same, you should have the same ratio. I seem to rememeber later Disco's might have had a different ratio, but the propshaft connection would be different by then. Of course, i might be talking total garbage and i'm sure someone will correct me if I'm wrong!
As you are suggesting, the disco diff might be a 24 spline if it's later. If it's a 10 spline it will be a straight swap and you won't need the half shafts. The 1988 90 is going to be a 10 spline. Just been there myself and ended up with an early Disco 10 spline axle so I could use my ARB locker that was already built into a 10 spline diff!