Cold Runner

[BradMph]

If the engine compartment has a radiator cowling, you may try taking that off to allow the engine to gain more heat.

What your describing also makes me feel like your gauge or sensor might be tweaked, giving false readings possibly. If your thermostat is opening to allow water into the engine, then the engine is reaching the temp that the thermostat is set to open on. I myself run a 180 with an electric thermo set fan, though others have their own choices like a 190.
You could try putting a temp gauge into radiator and this could give you a more detailed idea what your temps actually are. One thing about a thermostat that works correctly is that it will not open until it has reach a specific temp. If your motor isn't getting to temp via the thermostat blocking cooler water from entering, then your thermostat is probably staying open and defective. This is the only device that I can imagine that could cause an engine to not get to normal operating temps. Being located in Spokane where cold weather seems to arrive every year, I have never put a restriction in front of my radiator to keep the engine warm.

My gauge has done some strange things before also. It recently finally took a dive and shows that it's pegged. Though before that it was reading way low also. I installed an aftermarket gauge and it reads perfect. I think the fuel gauge may be on its way also because it reads a tick lower then normal on a full tank. 29 years is a long time for any car part to work correctly, so expect stranger things to happen in coming years.

Strangely these trucks can be sort of a self destruction when they over heat, having a truck unable to reach proper temp is a very rare thing to see in this forum.