Imprezza would be difficult I reckon, unless you could mate it to the fronty gearbox, and you might struggle with doing clutches every fortnight, as the scooby lump has loads of hp but lacks the low down grunt to get nearly 2 tonne of motor moving...
Easiest way would be get a remote/divorced transfer box, and the any rwd engine gearbox combo can hook straight up to it...
So could be a 24v 3.0 senator engine/box, small block chevy....anything really...would go sideways well on the road and do more front diffs than gallons of fuel off road!
Easiest swap in has gotta be a red top or maybe a c20let (turbo calibre lump)...they should go onto your gearbox as share the same bottom end as the 2.0i fronty lumps...