Again, don't forget you can burp the system to get an air pocket out. IF there is air in it, the thermostat will still open, but it won't circulate because the water pump blades are essentially spinning in an air pocket. Find some ramps, or some tall jacks, get the front all the way up as high and safe as you can and find the high spot in the system. Run the car until it's warm and you are sure the t-stat should be opening and open up the high spot, it may be the cap since it's in the middle of the radiator unlike a lot of other cars. It may be a bolt I can't find. But it's free and worth a shot. If you see a lot of bubbles coming up when you open the cap and the truck heats up then make sure you have some extra fluids on hand to put back in there so another air bubble doesn't get in.
Worth a freebee shot.