No, it is not supposed to slide. Sounds like one or both of the stops have come off or came loose. I remember seeing this problem discussed earlier. You should search for it. I seem to remember someone using some rubber hose and hose clamps to make improvised stops. The stops should be on the sway bar on the outboard side of the bushings on each side of the car.
98 evo 0 SVT #2999 of 6535 Silver Frost/Midnight Blue Lotsa bolt-ons