So you switch from cold to hot but get no heat?
First thing to check is if hot coolant is going through the heater core. With the engine warmed up and the heater on hot, open the hood and feel the two hoses that go from the engine into the dash. If coolant is flowing through the heater core, both hoses should be at the same or very close to the same temperature. If the core is blocked in some way you will have one hose that is hot and one that is a bit warm or cool depending on how bad the blockage is.
If you feel the hoses and one is hot and the other cold. You can try a reverse flush on the heater core. That involves clamping off the heater hoses, then removing them from the heater core and using a flush gun to blow water through the core in the opposite direction. It can clear out crud and let the core work again.
If both hoses are the same temperature then it is likely a blend door problem. The blend door sends air through either the AC or heater cores depending on how you set the temperature controls. When the actuator fails on them it no longer moves the door and you get air at whatever temperature it failed at.
As you say you are getting no heat at all I suspect it is the core or the heater control head. Your truck should have two different blend doors and they normally do not fail at the same time. You usually get heat on one side or the other. The pictures show the two temperature actuators and locations.
Please reply back with the results from feeling the hoses so we can go farther.