The heads 0331 (00-01) are notorious for cracking between cyls 3 and 4.

Ive checked the sending unit, replaced the oil, filter(s), oil pump/screen/tube.... i removed the valve cover. no cracks (visible). the problem all started after i ran synthetic in it once to "clean it out" for 500 miles.... shrug there is a guy with the EXACT same problem i'm having with his 2001 jeep and he's pulling his motor friday to find out the problem. hopefull his findings point me in the correct direction.