Hello,
I read your problem. From what i understand your truck is running hot on the freeway, even after replacing thermostat.There is several things that can cause a engine to run hot, besides the thermostat.
Here is a few things to check;
The radiator over times can get partially clogged, almost like jets on fuel injection system does. A lot of times we do not have the proper mixture of water and antifreeze which can cause breakdown of metals and small particles in side the coils and tubes of the radiator that will cause build up, almost like tarter on someones teeth. This of course will cause restrictions. Solution to this in most cases is a simple radiator flush. The service center of your choice , will flush the system by putting it under pressure and at same time can see if it is flowing smoothly and properly.
The other thing to look at is any restrictions of main hoses. From thermostat to upper radiator hose and to lower radiator hose.
The other thing, sometimes cause this is sporadic operation of water pump. Meaning working now and then. Can be worn out, of not putting proper water pressure to provide to the cooling system.
Another thing that can cause this running hot issue, can be a head gasket leak, even a small one can cause it to heat up.
Another thing that can cause engine running hot, is the fan not turning on to high speed when engine thermostat requires it. Meaning, that your thermostat is based on opening at a certain engine coolant temperature, then of course the fan goes in to high speed to help cool, and if the fan is not kicking into high speed then that will cause it to heat up or run hot. Have to test that when car is idling and then as it reaching the certain temperature, then the fan should kick on high. I would check fuses on this and make sure relay is kicking this on based on thermostat.
Conclusion: I think, that if you just flush your radiator and make sure the antifreeze and water mixture is correct, and fan is working properly, which any service shop can do this for under 100 bucks, this will solve the problem. I hope this helps Mike from fixya.