ECM power supply lost with ignition turned on...

[Chamberpains]

Somewhere along this circuit your probably gonna find a piece a wire rubbed thru, corroded, or not connected tight or your Batteries are weak and not coming up to 12v until the alternator brings the voltage up after the truck starts. Like Signature620 said : high resistance between the batteries and the 4 pin connector at the ECM.
You can easily check this circuit 2 ways. Get a multi-meter or 12v meter and unplug the 4 pin at the ECM and check the voltage reading at the 4 pin against the actual voltage back at the batteries. If it is less by any substantial amount like .2v or more you have that wire rubbed, corroded or loose. Also take your multi-meter or ohm gauge and Ohm out the ground wire using a good clean ground spot on the frame rail (the ground to the batteries is the frame rail so it gives you it's complete loop) If you get any high ohm readings then that wire is rubbed, corroded or loose. If either reading is off start the dirt fest and start tracing the wires from the 4 pin ECM connection all the way back to the Batteries or the other way around. Which ever you prefer. After you properly load test the batteries individually.

Also if you find either one of these wire with low volts or high ohms and don't feel like tracing the wire out for the break, you can just run a whole new wire from the batteries or frame. I've done this with a lot of circuit wires that I've found out of whack readings on and I didn't want to trace the connections out or it was just to minute to find where in the connection is going wrong.

**EDIT** I see the ground circuit end at Engine block ground (which does connect to the frame on most vehicles) But you'll want to ohm the ground out to this point instead. And I'm also assuming you have an ISX in your truck.