You don't really need that rear sway bar - you have a truck and probably don't drive like Mario Andretti. BUT - it is there for a reason: to keep the axle lined up under the body of the vehicle. Under certain (xtreme) conditions, the axle could move sideways on its perches. Front sway bars are far more important than the rears, and should always be attached (unless off road, of course... with it removed the rear axle can flex more.)

Also, by law, it has to be there. If you ever had to re-safety the truck that sway bar would have to be put back on, so don't toss it into the pond.

Scott