Idle Way Too High Upon Starting

Hello, or maybe I should start by saying: HELP! My 2004 Jeep Grand Cherokee, 6 cylinder, has started idling at 2,000 RPM when I first start the car. I can literally put the car in gear, and without stepping on the gas, away I go. What’s going on here? I’ve never encounted this problem before. Help! (Sometimes after a few seconds it will slowly go back to a normal idle, other times it will slow down, but not back to a normal idle even after drivng several mles.)

I know next to nothing about Jeeps, but the last time this happened to me was on a Toyota Avalon. I wasted gallons of expensive gasoline. It turned out to be one of the oxygen sensors. My “check engine light” was on but since I had just replaced an oxygen sensor, I thought it should have been OK. Turns out the mechanic had replaced the wrong sensor. Find a good mechanic, and support him/her!