The thermostat could be stuck open but if your truck gets up to temperature you should get heat .Like George said check to see if the coolant is circulating through the heater core by holding onto the heater hose were it goes into the core on the firewall and were it comes out .Some trucks I have seen have a shut off valve ,maybe check that .If the hose is hot going in and cold coming out you could have a plugged heater core .A small difference in the two hose temperatures is okay as the coolant will be a little cooler the way back especially if the fan is blowing air through the heater core.The only other thing I can think of is a air lock or low coolant .
