Idle Problems

You really can't turn down the idle on the 4.9. The computer controls it. It may be something as simple as the throttle body and plates needing to be cleaned. More involved but less likely is the ISC motor gone bad. I'd clean the throttle body first. FWIW YMMV DFB

I would look for a vacuum leak, most likely at the vacuum reservoir mounted on the firewall to the left of the brake booster, or the booster itself. Do the brakes fell normal? Does the A/C switch from defrost and heater vents to the A/C vents when the temp is changed? Listen for a vacuum or sucking noise under the hood while the car is idling. You can use a pair of needle nose pliers to pinch off vacuum lines under the hood to see if it makes a difference in the idle.

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