Can I put a used PCM or ECM in my 1996 Plymouth.

Flashing in the '90s was done for software updates, not to allow a computer to work in different cars. If your car has the factory anti-theft system, you can use an Engine Computer from any car with the same engine and year. If your car does not have anti-theft, you must use a computer from another car that also didn't have anti-theft. If you put one in with anti-theft, it will teach that programming to the Body Computer, then the car won't start until you replace both computers. That programming can't be undone.

Chrysler had very little trouble in the '90s with their Engine Computers.