The cracked head issue is really only common on Jeeps with the "0331" head casting. A hairline crack will develop around cylinder #4. You will notice a loss of coolant long before you will lose oil pressure from damage caused by the crack.
My vote is that you either accidentally damaged the oil pressure sending unit connector or wire, or maybe you knocked it loose and it fell off while driving down the road. If all of these look fine then look into a possible defective filter. Out of curiosity, do you use Fram filters?