Need an Educated Guess
 

1997 3500 Dually 4X4 Diesel. No charging so relplaced the Alternator. Next batteries showing no charge. Tach and Odometer quit. Is this in your opinion a bad ECM?Anything else to check or repair that may be the culprit. Thanks ahead of time!

Sounds more like crank sensor. It will shut down chargine. Make sure all terminals are in good condition. Also see if there is a blown (large fuse) in power distribution center. The crank sensor is mounted on front near vibration dampner. The likehood of failed PCM is remote. Suspect more good ones replaced than bad ones.

Probably the engine speed sensor or it's wiring. Sensor is located above the bottom engine pulley. Gap between the sensor and pulley should be about the thickness of a business card. Wiring to the sensor sometimes gets damaged or the connector corroded and sometimes the sensor itself just fails.
What's happening if the sensor doesn't detect any engine rpm the computer (PCM, no ECM on a 12 valve) thinks the engine isn't running. Nothing electronic will work.

What they said, also verify fuse #9 as it supplies the voltage for all of that.

I fixed this exact problem two days ago on my truck and it was the Engine Speed Sensor (sometimes referred to as crank sensor).

I had to get it from the dealer to get the correct part for my year but I believe also that Advance has the correct one for your year.

I believe the spec is .050 if you have feeler gauges.