I have two questionsabout my 2000 explorer that has 140,000 miles on it.?

The first is what would make my truck shake while in park but mostly when stopped at a light or waiting to fill up those are some examples.It idles high when you turn the heater on around1500 rpms is that normal?If my overdrive light does not work does that mean something bad or could it be a fuse blown or a bulb blown out ,my 4x4 switch light is out to so think they might be connected to the same fuse or same bulb maybe and if my transmission was going out what would be signs of that and how long will it last?I have never owned a truck and am not sure of alot of things and i really need some help.hoping to get some help,Thanx