why does my transmission cause my car to rock?

It sounds like the engine has a rough idle. I would start with checking for vacuum leaks and check the idle air control valve. If the check engine light is on, have the computer scanned for trouble codes. Most parts stores will do it for free.

when i start it up it idles around 1200 then settles at about 800rpm's.check engine light has never come on. it also makes like an "err" noise when it does it(kinda like the sound u get when u tighten a bolt REAL tight and it rubs on another metal). i kno its an older car so it may b something that needs to b greased or lubricated. i've checked around on the internet and every1's saying its eith a high idle or a tranny mount. could u maybe give me an estimate on how much either 1 wud cost me so i kno b4 goin into a shop (dont wanna get raked over the coals, im only 20yrs old and money's hard to come by these days lol)

The noise could be a bushing or something making the noise when the car rocks. I don't feel that a trans mount. That could make a clunk, but not cause the car to rock. As far as the high idle, again, you would need to keep your foot on the brake, but no rocking. I feel the idle is going up and down. There is an idle air control valve that is designed to keep the idle steady. If bad, one of the most likely symptoms is an idle that goes up and down. Thus, you get a surge in the engine which while stopped can cause the vehicle to rock. It could also be something as simple as a vacuum leak. If the car runs good once moving, the IAC is the first thing I would check.