You don't NEED them in the rear but it might be a good idea because unless you cut that little tab off the side where the bump stop is for the axle you will not get the 1 inch of clearance that is sugested. If you wana cut that metal tab off then no big deal. Also, the wheels wont line up with each other and your rears will looked sucked in compaired to the fronts.
As for the price, I got mine off ebay for like 55 or 65 bux a pair. As long as they are the billit aluminum then it should be fine. Don't use those cheap cast type.