motor mounts

[mimssmim]

hello, all
i have a 2000 pontiac grand prix gtp (supercharged). when i put it in reverse hold the brake and give it gas at the same time the rear of the engine moves a little more than it's supposed to, so im guessing i need the rear motor mount. am i wrong or right and what is the "official" name of the mount i need? because when i go to a parts store and tell them the symptoms my baby is suffering from they cant seem to tell me what i need but they can tell me what they have. thanks in advance for any information that can be obtained from this forum