Overheats at highway speed, with or without a/c

First, make sure you radiator fan is running (electric cooling fan).
Second, check your thermostat (or simply change it) as follows:
Drain the cooling system (petcock bottom of radiator)
Follow the top radiator hose back to where it connects to the engine.
The part with the two bolts in it that the hose connects to is the thermostat housing.
You may have to remove the hose in order to remove the housing.
When you have removed the housing, note the position of the thermostat before you remove it.
When installing, use new gasket, etc.
(you might be able to get away with not removing the radiator hose, but it is recommended)
Be prepared for a bit of spillage when you remove the housing.