Mine is a 2009 as well and I had similar issues when I got it. My guess is still that you need a heater core flush, pretty easy, though it was a bit of a pain to get the air out of the system after. You could try and bleed the system first to see if it is an air pocket. Google that for the 3.8.
Online vids make is sound complicated, but really all you need to do is find the two heater core hoses on the passenger side of the engine and disconnect them on the front side. Use a garden hose, (you will need an attachment that fits the core hoses)
Flush the core going each direction for a couple minutes. re-attach the hoses and make sure the coolant is full in the radiator and reservoir. With the radiator cap open and the appropriate coolant mix ready (I just get the universal stuff these days) start the engine and keep a bit of extra in the funnel. As soon as hot air starts blowing in the cab you are done. Shut the engine down, double check the fluid levels and take it for a spin. Check the radiator when the engine cools down to make sure its good.
If the engine temp shows warm and you are not getting any heat, there is air in the system. Shut down, check levels and close the radiator cap. Restart engine and give it a minute. For some reason that was the only way I was able to get mine to push coolant into the core.
Good luck, and be careful you are using the right coolant. Used vehicle like that, I would definitely say use the universal stuff. A bad mix can plug an engine.