eh er well it COULD be done but i dunno if it would be cost effective. i know they make rwd conversions for the focus. there is actually a special bell housing to mate the zetec engine to a mustang tranny. however to do this with the stock engine you would still have to pull motor and tranny and basically disassemble the suspension in front and rear. gotta cut the k member to make room for the tranny. fabricate motor mounts because u gotta turn the motor sideways to mate it to a rwd tranny. there is alot more to just getting the motor and tranny in and mounted but ill skip right to the fun part. because then you run the driveshaft down the exhaust tunnel. but you have to widen the tunnel and also give it more height so when you hit bumps the driveshaft doesnt hit the ground or the undercarriage of the car. that is lots of fun. then you have to find a ford rear end that will fit under the focus and figure out how to get that in there. a 7.5 would probably fit nice. 8.8 probably way too big. i mean if you got the time and patience for something like that then thats awesome and i want to see it when you're done and ill help in any way i can but it isnt something that i will ever do myself unless i randomly come into alot of money so i can just drop it off with somebody throw about 20 grand at them and come back and get the car when its done.
Actually after doing the research it's not all that bad if you keep it to doing the rear end as a solid axle. But in the end if your gonna go RWD you might as well do a V8 at the same time. Heres all the sites i found for the kits and some FAQ's about doing it all
and if you wanna go with a IRS you can use the rear subframe of the early escapes for the basic setup, but I'm not sure there's a LSD kit for it so here's another site for something to be able to do that