I did a search, found a couple started threads that never came to a resolution.

I have a 2002 with the 4.0. Within the last month or so the truck has developed a habit of dropping to extremely low RPM levels when shifted from drive into park. There are a few other "clues" that may be of assistance in uncovering the cause of this failure. When the truck is dropping RPMs, there is an whining sound that accompanies the truck's death. Also, if the A/C is turned on, the truck dies much less frequently, as the RPMs maintain a higher level. Lastly, if I tap the gas pedal as the RPMs are dying, it corrects the problem, and the truck won't die.

The problem tends to occur after 15-20 minute trips of stop and go driving, and I am not slamming the truck from drive to park. In fact, I generally wait about twenty seconds between parking, and shifting into park, hoping that the truck will be able to make the change without dying. Any help would be appreciated, as I have no idea where to go from here.

The truck may need a tune up, as I have owned it for about 25k miles since buying it (currently ~90,000). Any other suggestions would be greatly appreciated!

 

It may be time to clean your IAC valve.

 

that would be cool. just throw it in park and its already turned off. one less thing to do.
lol
clean iac and maf. may help

 

Check the IAC valve Ive also seen a bunch that suck the PVC tube shut..it runs along the back of the intake from one side to the other. Over time it just gets old and sucks shut or develops cracks and can cause that exact problem.