GMC Troubleshooting Questions? Ask a Mechanic for Answers
Ok, first things first. The security light. Is it on also when cranking the engine? If so, can you try another key?
Do you have a code reader or scan tool to retrieve codes? If so, do so.
Now, about the not continuing to run. Check if these fuses have voltage with the key on just run. The ECM 1 fuse which powers the fuel injectors and the crank sensor, vcm, as well as the ignition control module and the ignition coil.
Here is a diagram of the Engine Performance Circuits, can I assume the wiring harness for the engine, etc is from the Jimmy?