i went out and did some serious wheeling yesterday on some very difficult trails and i kept noticing this knocking noise coming from under the jeep...i have a 3.5" lift and i was thinking maybe it was the driveshaft hitting somewhere? i didnt really notice it when i was flexing with the wheel straight it was mostly when i was flexing with the wheels turned to either side...

 

I know, one of my bumpstop extensions arent quite mounted straight, and when the spring compresses, it acually hits the bumpstop making a knocking noise.. maybe you have the same issue??

 

If it is only when you are turning and flexed up I would guess it is a tire hitting someplace, and you are hearing the lugs rubbing.

 

Check and see if your track bar is hitting your diff cover. Very common when doing some major flexing. It will be more like a single clank then a rubbing sound. Look for paint scrapped.