Depending on your set-up, one benefit for mounting the shocks behind the axle is that during extreme acceleration (drag launch) the axle tends to want to rotate top forward. If you are using the stock leaf spring set-up mounting the shocks behind the axle will help dampen the torsional effects by acting in compression.
If you have a 3 or 4-link set-up the upper bars restrict the rotation, your shocks are really only dampening the effects of bump oscillation. Remember the springs do all the work, the shocks dampen the rate of how fast a spring compresses or extends to control wheel hop, squat or dive.