idle increases in park

XJ is a 1990, 4.0L, AW4, 242, 90,000 miles. The problem is when the truck is shifted into park, the idle increases over 1000rpm's (sometimes up to around 1400). It usually does this when it is up to operating temp. The idle is fine (about 700rpm's) when it is put into forward or reverse. A little other info on truck: recent tune up, modified "level 10" tranny, O2 sensor at 50,000 miles (the emissions maintenance light is now on, but the book says it will go on around 82,000 miles and is only a reminder to change O2 sensor) , and I have spares for all sensors under the hood. Any info would be appreciated, thanks, John Burke

check your egr/evap purge solenoids for leaks

check the intake manifold bolts for tightness.
Check for any vacuum leaks.

Check the contacts on and remove and clean the IAC (Idle Air Controller) attached to the throttle body. It has a small plunger type valve which controls idle. It is controlled by the computer.