It sounds like the timing chain needs replaced. I would remove the timing chain cover and inspect.
Here are directions for replacing it:
2000 Jeep Truck Grand Cherokee 2WD L6-4.0L VIN S
Vehicle Engine, Cooling and Exhaust Engine Timing Components Timing Chain Service and Repair Procedures Timing Chain
1. Disconnect negative cable from battery.
2. Remove the fan and shroud.
3. Remove the serpentine drive belt.
4. Remove the crankshaft vibration damper.
5. Remove the timing case cover.
6. Rotate crankshaft until the "0" timing mark is closest to and on the center line with camshaft sprocket timing mark (Fig. 58).
7. Remove the oil slinger from the crankshaft.
8. Remove the camshaft sprocket bolt and washer (Fig. 59).
9. Remove the crankshaft sprocket, camshaft sprocket and timing chain as an assembly.
10. Installation of the timing chain with the timing marks on the crankshaft and camshaft sprockets properly aligned ensures correct valve timing. A worn or stretched timing chain will adversely affect valve timing. If the timing chain deflects more than 12.7 mm (1/2 inch) replace it.
Assemble the timing chain, crankshaft sprocket and camshaft sprocket with the timing marks aligned (Fig. 58).
1. Apply Mopar Silicone Rubber Adhesive Sealant to the keyway in the crankshaft and insert the key. With the key in the keyway on the crankshaft, install the assembly on the crankshaft and camshaft.
2. Install the camshaft sprocket bolt and washer (Fig. 59). Tighten the bolt to 68 Nm (50 ft. Lbs.) Torque.
3. To verify correct installation of the timing chain, rotate the crankshaft 2 revolutions. The camshaft and crankshaft sprocket timing mark should align (Fig. 58).
4. Install the crankshaft oil slinger.
5. Replace the oil seal in the timing case cover.
6. Install the timing case cover and gasket.
7. With the key installed in the crankshaft keyway, install the vibration damper, washer and bolt. Lubricate and tighten the bolt to 108 Nm (80 ft. Lbs.) Torque.
8. Install the serpentine drive belt.
9. Install the fan and hub assembly. Install the shroud.
10. Connect negative cable to battery.
Let me know what you find or if you have other questions.
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Friday, October 9th, 2020 AT 9:11 PM