I think I have traced down the cause of my problem. I put a volt meter on the wire that goes from the park/neutral position sensor on the transmission to the ECM. I'm getting 0 volts whether the trans is in park or reverse. I'm thinking there should be a signal on that wire when the trans is in park to tell the ECM it's safe to start the engine. Unfortunately, I short on cash right now and that sensor is $100. Hopefully I can get one next month. Once I've confirmed that is the issue I'll post an update.