Check your exhaust pipe where it passes over the cross member near the cat. You should see a hanger/bracket that looks like the "L" shaped thing in the photo. Obviously, this is a replacement part where the clamp fits around the pipe and the rod inserts into a receiver mounted on the right (passenger) side of the trans tail shaft - atop the cross member. The original equipment consists of a rod welded to the pipe.

 

This happened to me right before my exhaust fell off.

 

Well, I had taken the Jeep off road over a lot of washboard roads. I found I lost a nut on my passenger side motor mount, and a nut off of the exhaust mount near the transfer case cross member. I replaced both parts and voila, no knocking noises. Thanks for all the suggestions and help.