Is it possible to remove the heat shield from the catalytic converter on 1998 Jeep Grand Cherokee?

Sure it is possible but it is not advisable. The heat shield is there to prevent heat from the converter from potentially getting the floor of the car so hot as to cause a fire. If it is rattling, a temporary fix is to purchase two large adjustable metal clamps and simply place them around each end of the converter and heat shield. This will stop the rattle, and protect the underside of the vehicle.