Actually, when you remove the bumper mount you have a perfect place to attach a mounting plate for your servo. You will need a heavy duty servo saver and a nice strong servo. Now from there you need to remove the stock tie rods and add some longer ones to reach the servo horn. That is really all it would take because the rear is configured very much like the front as far as the hub carriers are concerned. Now as far as running with 4 wheel steering, it is way touchy. You probably want a radio with a mixable 3rd channel. A splitter would work but that would put way to much steering in the rear. A 10% to 15% mix is all you would need to get around those tight turns.
Hope that helps! I can do some measurements and a parts list if you really want to consider it.