What are you going to do about the ABS/DSC system? They are not the same from a 2wd to a 4wd.
Maybe it's just me but why not clean up the 4wd BMW and sell it? Then buy a good used 2wd BMW. The market for a 4wd BMW is usually not that bad in colder climates. Since there are tons of 2wd BMW's out there, you can take your pick.
Even a friend of mine bought a theft recovery 2003 330i last year for $3500. It needed a hood, front fenders, bumper and headlights. So for around $2000 in parts and some paintwork, he has a really nice car. A $5500 6 speed 330i with $75K on the clock seems to be a pretty good deal to me. It has a salvage title but he'll probably keep the thing so long that it isn't going to make a difference. Now he has a car that he can spend some money on and do some mods.