My Toyota 4 runner idles around 700 but drops to 500 in gear with check engine light on what could be wrong?

Check engine light is a problem sensed from the computer. You need to find a code that will tell you what the problem is. A local parts store may do the test for free. There is a way to do it at home but I need to look up the way to do it. The problem will be evident once you find the code. Mine is usually a connector that is loose or off. Some sensors being bad will set limits as to what the engine will do. Like the knock sensor will reduce power by up to 50 percent. So check out the code.