You can mount it on the back.  It might not be that simple though.  You would probably have to shorten your frame more because you already have your shackle mount set up under your frame.  If you added mounts to the back how it is, you might not even be able to get the shackle to bolt to the spring and mount, and if you could, your angle might be bad.  Some good things about mounting it on the back of the frame instead of under:
1.  It will lower your 4Runner down a bit.
2.  It eliminates the possibility of inverting your shackles on the trail, if you set it up right.
3.  It is unique, and therefore cooler.

On our Formula Toy we've got the mount on top of the frame and the shackle curves around the frame.  Sort of like a "C".