It's been working all day today and yesterday. That hasn't happened in quite a while. I won't declare victory yet. I'm going to wait a week and if I get the CEL to turn on every time I will do my happy dance. Here's my theory of the problem:

The gutters on the right fender under the hood have been clogged for years as evidenced by the crud buildup behind the rubber cushions. The water goes someplace- right to the area of the main relay. This moisture rich environment promotes corrosion, not only of the MR's pins but the sockets in the connector.

By replacing the Main Relay and then removing it a few times to attempt diagnostics, the sockets in the connector got cleaned.

Now the truck starts. Every time.

Anybody know of a product to spray in the connector to prevent this from happening? I know WD-40 drives water away but it can't be that good of a conductor.