can you put a pcm out of an identical grand cherokee and have it work in another?

You should not need to ever replace a PCM unless you put it under water. Diagnose your vehicle or have someone that knows cars do it.

I think you would need a reflash.

thanks for the help,i did have the pcm diagnosed as the probable problem. I dont see why one mechanic would find it hard to believe that a 20 year old computer would malfunction. In my experience, anything can break.

If you put the battery terminals on wrong it will fry the pcm

If you replace the pcm, key barrel, immobiliser (5wk4 812c) that have all come from the same car then you shouldn't have to reprogramme the new pcm. Note that the new pcm will be coded to old car therefore it will have the old cars VIN number stored in it.

Having the old VIN is not a problem?

Yea you can but have it reflashed https://mintt.com/jeep-pcm/