2003 Jeep Grand Cherokee P0432 Fault: Rear Cat Converter Issue and O2 Sensor Diagnosis

I own a 2003 Jeep Grand Cherokee with a 4.0L engine, two-wheel drive, and over 200,000 miles. Three weeks ago, the Check Engine Light (CEL) activated with code P0420, which indicated a catalytic converter efficiency issue. I had a mechanic replace the front catalytic converter, after which the code cleared. However, three days ago, the CEL returned and now shows P0432 — this is specifically related to the rear catalytic converter. I recently purchased a direct-fit replacement for the rear cat converter from Napa and installed it this morning. The code cleared temporarily, but after driving five miles, the CEL reappeared and the P0432 code has returned. To date, I've spent over $2,200 with two different mechanics, yet the issue persists. Previously, I also experienced a P0112 code, which was resolved by replacing the throttle body, IAT sensor, and associated wiring to the PCM. I'm seeking expert insight into this recurring fault — especially since the P0430/P0432 codes are often linked to downstream oxygen sensors or aging upstream sensors. Any guidance on proper diagnostics or repair steps would be greatly appreciated.