I have a 2005 jeep liberty that overheats when on highway speed with the AC on. I bought this jeep used and also noticed that the coolant seemed to be a little below the cool mark when the engine is cold I don't know if that would be a cause to this. Could it be the thermostat?

It depends.
Were you towing?
What was the weather like when you over heated?

I've completely removed my fan and clutch. When driving down the highway, not towing, and AC on, in florida summer, and I have never overheated. But mine, and most CRDs, have an electric fan in front of the radiator.

Now, if your fan clutch is factory original, without a doubt it is bad and has been for a long time. So a better question is if your electric fan is working or not.

Electric fan is an easy test. Let the engine idle, turn on the AC, stand in front of the grille and listen for the fan to come on. If it doesn't, then you have likely found your problem.

It is possible for you thermostat to be bad also. If fact, if its the factory original thermostat, it probably been bad for a long time. But it typically fails in the open position and does not lead to overheating.

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U.S. Army Retired

it is in the hot summer of texas. the electric fan turns on and the fan clutch seems to be working as it does spin. idk if the thermostat is bad though. I am not towing anything. i'm just cruising down the highway at 75 and it starts to climb. I turn the AC off and it goes down and when I turn the heater on it goes down faster.

You probably could benefit from a new Hayden heavy duty fan clutch. Lots of discussion here about it. Just search the forum for the part number.

But I would also take a good look at your cooling system.
Make sure there is no dirt or leaves blocking airflow through the radiator.
Top off the coolant tank and do a pressure test for leaks.
A cooling system flush couldn't hurt either. Hopefully the previous owner always used the proper coolant in the radiator. Using the wrong coolant can cause the radiator to become clogged.

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U.S. Army Retired