Having problem with idle.

I have been having idle purges, while in drive. I'll be sitting at a traffic light, or at a stop sign and the idle will be higher than normal, if I let off the gas, it takes off on it's own. If I put the Jeep in park, neutral, it will shoot up to a steady 1500-1800 RPM and won't come back down until I put it back into drive. Still when in drive it's high. Did a tune up about 5-6 months ago (new plugs, new cap, new rotor; didn't do the wires because they're 8mm MSD and look pretty good). Specs are: 99XJ, 4.0, AW4, 231, 30/8.25, AIRAID intake, banks header w/ an obvious crack in it, high compression head (not sure what kind but that's a whole nother story). Haven't changed any of the sensors, just now looking into the problem. I was told to check for vaccume leaks but I'm not sure of the best way to look for them. Any input would be appreciated. TIA.

-Andrew

spray down the intake manifold while running with brake cleaner and see if that changes the idle at all. Im thinking its a vacume leak.