I don't know much about yj's, but I would guess it's your ccv or pcv valve (whichever your jeep comes equipped with. Start by checking the hoses/elbow that come off your valve cover, a lot of times these can get clogged up causing blow by. Disconnect the hose that run from your intake to the valve cover (on the intake side) and start the jeep up, there should be suction in that hose... if not, it's clogged and needs to be replaced.
Also check for air leaks around the oil dipstick, the oil filler seal at the front of the valve cover, and also the grommets where the two elbows go into the valve cover. Any air getting in these places will allow fumes to blow back into the air box.
I think shot piston rings can also cause the issue. If you can't find anything wrong with the ccv/pcv system or leaks in the other places, you might want to run a compression test on the cylinders.
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