is it a constant knock that slows with speed, or is it constant with the jeep's engine?
I had front drive shaft joint go bad that you couldn't hear while driving but as you coasted and slowed you could here it "knock, knock, knock" as the drive shaft went around slower.
You can go out and grab your front driveshaft, and push up and down on it. if it moves at all (sloppy play wise) at the joints (the double one close to the transfer case, or the other at the front axle) then that may be it.
you can check the rear one the same way...grab the driveshaft and push it up down, left right at the joints and see if anything moves