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Sounds like you have a misfiring cylinder. The vehicle is most likely misfiring in park also but the reason why you dont feal it is because there is no load on the engine. Whena load is put on the enigine the engine is working much harder thuse you are know feeling the misfire. In most cases you just have a bad ignition coil that sits on top of the spark plug. If the check engine light is not on then it is hard to figure out which cylinder is misfiring without using a scan tool.
Click on the image for a picture of the coils: