Since you have shackles on the axle, it isn't fixed either side to side, front to back, or against roll. Gas shocks won't help that at all. If you went to the gas shocks, the first time you hit the gas hard, the pinion would probably rotate up, bottom the shocks, and probably break or bend the rear spring.

In fact, if you have shackles on both sides of the axle, your rear spring won't prevent side to side movement either.