It is most definitely do-able. In stock form the rear axle has the same spindles, knuckles and steering linkages as the front. It just locked in the straight position by a screw into the axle housing. To make it rear wheel steer just mount a servo on the TVP frame and run a link to it, or the most simple way is to grab RCGuy's servo mount for the axle and put the servo directly on the axle. Everything's there to make it rear wheel steer, but do I recommend it...no. But that's me. I did the complete opposite and removed all the steering parts from the rear axle and locked the axle ends in place with RC4WD's straight axle kit to eliminate all the slop in the rear end from those links and knuckles so it was more stable at higher speeds.
Here's some pics....
STOCK rear axle
After with RC4WD's axle ends...