Does the 93 only have one O2 sensor before the CAT or is it an OBDII with a post CAT O2 sensor as well? I would think "dumping fuel" would trigger a MIL light unless the O2 sensor is bad. I also understand a plugged CAT would cause major loss of power, especially under load, but as stated the source of the heat is probably an overheated CAT. A vacuum test can help diagnose a restricted exhaust but I'd be more interested in an exhaust gas analysis over an engine scan unless the scan included it. If the shop has a dyno even better as the engine can be analyzed under load. I know removing the CAT is no small job so I'd spend some money on analysis first, but the CAT doesn't owe you anything given the miles and age of the truck. I just replaced my CAT during an exhaust system change but there was nothing wrong with the CAT, it was just preventative.