I don't do a whole lot of highway driving, very little actually, just a short jaunt to work in the morning at 80km/h (posted limit, but we all know that isn't the speed I'm going :P)
The cost isn't a big deal, I have access to lots of wreckers and I'm sure I'd be able to find a ruined transmission, its just whether or not it'd be possible.
And as for the final drive, what makes them different? I did some searching and they appear to be the same, at least cosmetically. The four mounting bolts are in the same configuration and the shaft appears to be the same size. Is it the spline onto the wheel that differs? Or something else?