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Jeep Grand Cherokee DIED suddenly

So, my boyfriend and I went 4 wheeling in tall grass in his 96 Jeep Grand Cherokee Lardeo 4.0, I know, NOT a good idea. The next morning we left to drive about four hours and forgot to check the undercarriage, REALLY not a good idea. We were about 2 hrs in and the transmission started slipping on the highway at about 70 mph. We stopped for gas, it started up fine, and on we went. Transmission was acting up over 70, but seemed fine at 65. He told me to pull off so we could check the fluid, I exited, turned left, and the Jeep died at about 45 mph. Just shut down, no gauges working at all. Cranked but would not turn over, but you can hear the fuel pump come on. We looked under and found grass around the drive shaft that had ripped out the downstream O2 sensor. Of course it was late, no parts stores were open, and we left the repair manual at home… He tried to stick the wires back in the sensor but that didn’t work. We got towed home, replaced the sensor, and nothing changed. Checked for fire at spark plug, nothing. We tried to get the check engine light to flash the codes manually, and that didn’t work either. No check engine light at all. We went to checking sensors with the volt meter, trying to make the check engine light come on. Nada. Very low (about half of normal) readings on major sensors. We had it towed to a shop, they said it was reading crazy and must be the PCM. We have tried two different ones programmed for the vehicle and they didn’t change a thing… Still cranks, no start, no gauges, any ideas? Thanks so much!