Hello and welcome to Just Answer!
Yes it could be your thermostat that is causing this.
If your temp gauge is still up to the normal range(where it usually sits) then I would suspect your thermostat is still ok. I always check thermostats with a scan tool, you can see what the coolant temperature is by looking at the coolant temp sensor reading. The thermostat should open around 195 +/- 5 degrees. When the thermostat opens you can see the temp drop, so just before the temp drops that is when it opens. This is a far more accurate way to test thermostats.
Another thing you could do is feel the heater hoses and see if they both feel hot. they should both be hot with one a little cooler.
The heater cores on these trucks are a very poor design, they are very small and have small cores. These cores eventually get scaled up and reduce the cores ability to disapate heat into the cab. I have replaced lots of heater cores in these trucks.
I also use an infared temp gun sometimes to measure the heater hose temperatures to see how hot they are. If they are hot then you either have a heater core that is getting scaled up or a blend air door that is not working properly.
Your blend air door controls the amount of air passing through the heater core and if it is not closing all the way it can allow some of the air to bypass the core and would cool the outlet temp down. I hope this info helps you, let me know! Thanks,