One more thought: Determine if the problem is caused by poor airflow (a clogged AC evaporator, for example, as previously described) or not enough heat from the heater core itself. Could be a combination of the two.

For a suspected airflow reduction, select defrost and play with the fan speed. You should feel a marked airflow increase as the fan speed increases. It's hard to quantify what is good airflow, but it's quite strong, especially from the defrost vents.

The other possibility is not enough heat transfer at the heater core. This could be due to low coolant temperature, poor coolant flow, or scale inside the core. Check the coolant temperature at the heater lines using an infrared thermometer. With the engine fully warmed up, you should see at least 180F or so at the inlet line. This is the line connected to the engine just upstream of the thermostat. If not warm enough, the thermostat is opening too soon.

To check the heater core for proper heat transfer, compare the line temperatures with the fan on high. The return line (connects to the water pump) should be at least 30F cooler than the inlet. This normal drop means heat is being transferred to the cab. If less than a 30F drop, you've probably got scale built up inside the heater core.

Two scenarios for scale buildup:

1) Severe buildup will reduce coolant flow, resulting in poor heat transfer.

2) Light buildup still lets coolant flow, but acts as an insulator. Hot coolant in, hot coolant out.

In either case, you'd see less than a 30F drop from inlet to outlet.

 

I’ll check all of it. Thanks for following up, kr98664. I never would have figured all of this out on my own.
I very much appreciate yours and everyone’s responses.