My ZJ had this problem, turned out to be a cracked elbow on the PCV (CCV) valve.

 

I will try this on mine. My pcv elbow is a little gunked up looking.

 

Hey guys i have the same thing high whistle when i let off the gas. I have a feeling its the 249 full time t case. It starts grabbing in corners after highway speed and i start getting the sound.are you also running a full time case?

 

Ya but I can hear the whistle coming from the under the hood. Once I was able to get out n pop the hood fast enough n could hear it coming from around the throttle body...

 

Ok ya o had that happen to my xj but i was lucky i found a few loose intake manifold bolts

 

Ok so I have been reading about and dealing this whistle now for some time. This week I took a big piece of hose about 3ft long, put an ear plug in the other ear and used it to locate the area of the whistle. It was up underneath the intake and sounded like it was coming from the gasket area so I took a chance.

I tore it down Wednesday, Changed the gasket and reassembled it. THE WHISTLE IS GONE! I have driven it several times now and NOTHING! FANTASTIC is all I have to say, I am a happy camper right now!

322,000 and running as good as the day I bought it. 2nd Cherokee I've owned that's hit 300,000+. With a little TLC now and then.....They go a long ways!

 

I just wanted to pass this on. To find vacuum leaks, you can use a hardware store propane torch. Don't light it, just crack the valve open a little and play the gas around your suspect areas. The leak will suck the gas in and cause the engine to speed up. I found my cracked CVV elbow this way. Also, a spark from the wires will cause the torch to flash or pop.

 

jjsg09:

Where did you find the intake crack? I have the same issue.