2003 cts ecm
 

I'm having major electrical problems with my '03 CTS. The tech says it needs a new ECM. I've found a salvage one that is considerably cheaper than a new one, but none of the numbers on it match. I've been told that they don't necessarily need to match as long as it is out of the same year and model because it needs to be reprogrammed anyway. Is this the case? Also, is an ECM the same as an ECU?
Symptoms: fuel gage does not work, key cannot be removed, will not shift from park.
Thanks, RB