I obviously haven't heard the knock, and don't know your financial situation or future plans for the Jeep, but if it just started and is only knocking slightly, I'd be tempted to pull the pan and check the connecting rods to see if you can find a loose one or more. If you find one, remove the cap and inspect/measure the crank. It might be possible to just install some new rod bearings and run it for a while until you can afford to do a 4.0 or V8 swap. I dealt with a motor one time that had a knock that was traced to a bad rod bearing. The crank journal measured about 0.010" under, so we polished it up and installed a 0.010" over sized bearing on that one rod. The motor ran fine for a long time.