I had a Dodge Caravan (1988) that smoked at idle when at operating temperature. That was also a Mitsubishi engine, and they were known for the valve guides/seals going bad.

With 90,000 miles on your truck, I'd look into replacing the valve seals. It can be done without taking the head off. I'm not sure what to suggest to test for the seals...perhaps one of the experts knows a trick.

And I, too, hope you're not contending with a cracked head.