i think pulling the belts is a good idea. I had a pretty bad vibration once due to a bent fan; finally broke itself off.
Could you explain how this started again?
This was happening from the time you bought the jeep? Did the PO mention this or can you contact him?
As far as the balancer...so you already replaced a known good one with another good one, and it failed?
I would def. diagnose the problem before jumping into a rebuild. It may be something simple. As far as the knock goes; don't condemn the engine yet. You may be hearing something else.
Or you might have thrown a rod bearing and thats the noise and vibration.
Pull all the belts and see what that does.
You can also get an idea of what each cylinder is doing by pulling one plug wire at a time at idle. If the knock goes away while one of the wires is off, thats generally an indication of a problem with that cylinder. Usually a rod bearing.
You can also either use the oil off the dipstick or drain the oil and check for bearing material.