Engine runs rough until warmed up. But it only does it when there has been a lot of rain. Doesn't matter if the truck is driven in the rain or not, still does it.

Ideas?

Common problem. You need a distributor cap and or sparkplug wires and coil wire. They break down after a while and moisture promotes crossfire. Goes away when engine warms up and drys up moisture. My 2001 V6 does it now. I'm waiting for warm weather to replace stuff. I don't have a garage.