93 Blazer Gauges go Wacky!
 

1st off I'd say ignition switch.
2nd you'll probably find the answer or similiar issues others have had if you use the Search feature at the top.

does it crank hard because mine use to do that and then shortly after battery went out, if ur battery isnt holding voltage after u shut it off it would cause low voltage. and that could mess up ur guages soo idk its worth a try try using a multimeter on ur battery next time before u start it if its under 11 volts its prob ur battery

So far we have replaced the alternator, battery and the complete steering column. The gauges still bounce when the key is turned on.

My gas gauge will bounce around. But it hasn't worked in 3 years. You say it only does this for the first ten seconds of the ignition switch in the on position?

Could the ecm be going bad?

Welcome from New York.