Need Help with 1999 Jeep Low Idle Problem?

[Alainn]

I've had a 1999 Jeep Grand Cherokee Limited for six months. It's a 4.7L with 80,000 miles on it. In park it idles between 500-600rpms and sounds a bit rough. When in drive and stopped at a light, it lowers to 300-400rpms and, every once in a while, just out of the blue, will dip even lower and almost die. It has died on me once doing this, but started right back up. If I've got the air on or the defrost, it will idle at the 500-600 rpms in drive, sometimes dipping like it wants to die. Also, when first starting it in the morning it will rev up to over 2000 rpms which seems a little high. We have cleaned the throttle body and put in all new plugs. We have no codes showing and even took it to a Jeep dealer where they put it on the diagnostic but nothing showed and they refused to look into the problem further, of course charging us the eighty bucks to tell us what we already knew. Transmission seems a little rough at times, too, and wondering if there might be a connection. Replaced both sensors in the transmission as well as adding some Lucas transmission fluid. Could the low idle simply be the Idle Air Control sensor? Any help would be appreciated. Thanks in advance.