Can I swap the good ECU from my 93 5 speed (the X I'm having a problem with is an automatic)?
Yes you can swap the ECUs! The auto trans doesn’t have any control from the ECU. So no issues there and no trans codes either
That's technically not true. There is computer control of the tcc lockup and 3-4 shift. However you shouldn't have an issue with swapping the ECUs just for testing.
Off the top of my head, did you match the calibration on the old ECU with the new one? There were specific calibrations for California emissions, unless your truck was NOT originally bought in California.
Nonetheless, it is doubtful the new ECU is bad. You might have a vacuum leak somewhere. Its very rare to have both O2 sensors go out, I doubt they are bad.