2004 ford explorer shaking while in park and idling?

I have a 2004 for explorer with about 137000 miles on it and it shakes pretty bad when its in park or when it idles. As soon as i hit the gas the shaking will stop. The check engine light is not on and besides for that there really isn't anything else wrong with it. It shakes pretty violently though. I just need to know if anyone has any idea of what it could be.

Added (1). all of this is good input. But i won't know how to do any of this myself… So are these all easy fixes or cheap? And roughly how much am i looking at spending. Thanks for all your input.

Added (2). and it idles fine. The RPMs are not jumping around or anything… It idles around or just below 1000.

Your idle speed is probably to low.

Either an idle speed adjustment or probably needs a tune up.

1 of 2 things, ck 4, vacuum hoses off or lose, see if idle spring is off the linkage. Ps might even have bad motor mounts. This is easey to see. Have hood open have someone gun engine a little, to see if motor raises up.

As you can see… It can be any number of reasons.

Sure do, You probably have a bad motor mount. 2 people can confirm this. Open the hood and have someone look at the engine, apply brakes firmly and go between reverse and drive slowly and you'll see the engine moves more than it should when it rocks to one side. It will be the opposite side mount that will be bad… If it moves a lot to the left, the right mount is bad. It's not a hard job to replace them. You didn't say the idle was low or erratic.