well you need to eliminate a few things.
First, are you going by the stock gauge? if yes, use an infrared thermometer and point it at the thermostat housing to make sure the gauge is reading correctly and you are infact overheating
If you dont have an infrared thermometer, Get one! they are invaluable
OEM/Non-Contact infrared thermometer (25245) | Thermometer | AutoZone.com
second, do you have white smoke(steam) coming from the exhaust? is there coolant in the oil or oil in the coolant, no? okay
is there exhaust getting into the cooling system?
autozone has a tool you can rent called a sniffer test and it uses a blue fluid that turns yellow when you draw exhaust through it.
if you dont have any of those problems then that rules out the head gaskets.
Third, the thermostat. it takes 15 minutes and some permatex water pump gasket maker to check to make sure its in right.
To check to see if its working properly, throw it in a pot of water on the stove. use your infrared thermometer to check the water temp and at 190* it should open fully.
Now that you've eliminated three possibilities
if it isnt any of the above and your cooling system has been properly maintained. My guess would be the stock 3.8 radiator isnt up to the task of keeping the 5.0 cool, and you should upgrade the radiator.