you need to start with the basics on this one. When it's raining and won't start you need to check for spark, fuel pressure and injector pulse. Check to see if you have good spark at the coil and then check to see if you have good spark at the plug. You could have a weak coil, a bad coil wire, bad spark plug
wires or poor cap and rotor condition. Any one of those things would be affected by moisture in the system.
Verify that you have the required fuel pressure for your truck. Fuel pressure spec is 44 to 54 psi but the spec is with the engine idling. So, in most cases key on engine off it would be slightly higher.
Injector pulse could be checked by unplugging an injector and use a test light between the two contacts in the harness connector. If the test light flashes as you crank the engine over it means that you should be getting injector pulse. If it doesn't flash you may have a cam or crank sensor problem, a pcm problem or something between those problems.
anyways check to see if you have all three things working normally and if any one of those things are not present when you're cranking you know what to concentrate on.