My 00 XJ (60k Miles ish) has a knock coming from the Flex plate area. I thought it was just the bolts, they needed a little lightening but not bad. Still didn't fix my problem. I made sure to watch a bunch of videos on the issue, but everyone who ends up having a bad flex plate the knock is sporadic, whereas mine is in tune with the RPMs. the more i rev the more it knocks. I'm pretty sure its not the motor because I used my hillbilly stethoscope (a shop vac hose), but I've been known to be wrong every once in a while. Anyways, here is a video of my jeep and its noise. Let me know what you guys think!

 

you can see the flex plate wobble i have the same issue. replace it and be done.