The car is shaking badly and acts like it's going to cut off anytime
service engine light in on on 2004 Mercury Mountaineer
sounds like a missfire. have the code checked and find what cylinder is missing. get it checked soon so you do not damage the cat converter.
if you read the manual, it will tell you that the service engine light comes on from bad gas or fuel cap not on tightly, most likely the shaking is from bad gas.