Began doing tons of research on that Focus....There is a kit to bolt a rear end on the car.
All the mounting brackets needed to bolt the new axle and rear diff to the stock Focus shell will be supplied in the kit.
I found this. I have however not been able to find very detailed doc's on the actual installation, IE did they reinforce the shell? And also how long can this last? The focus can be converted to RWD for under $60, 000...However, seeing as how I am going with a smaller engine (that can pull a Buick to 10 seconds and less), and won't have to do suspension upgrades to the front right away, things should work out cheaper...

So any thoughts on bolting directly to the unibody? Can't find if they used a sub frame or not...
Oh, and here's the RWD kit for the Focus....maybe useable?
http://www.kugelkomponents.com/focus/focus.html

Again knuckledragger, thanks for finding me the idea.