Wierd knock in rear

One day I was driving to a friends house and I turned on his road. All of a sudden I realized a strange knock in the rear of my XJ. When I stopped to listen, it sounded like loose torque converter bolts would sound, except louder and more rough (I can hear/feel it easily in the driver's seat.)

It doesn't do it all the time, in fact not very often. Yesterday, I put her in reverse and it came back, then through her in drive and it went away. It is not consistent at all, is decently loud and vibrates the jeep a little, and happens out of nowhere at any time the engine is running; driving or idling. I am suspecting maybe something loose in the rear differential??? She drives great with 152,000+.

Any help would be excellent

Have you looked at the U-joints or other driveline components?