Always get the actual Pxxxx codes from Autozone if you can. The folks around here know their stuff and can often diagnose the problem accurately the first time with the codes vs. Autozone's interpretation. I second the idea of a cheap code reader. I have an Actron that cost me around $40 and it's invaluable!
Regarding your temp/coolant troubles, I suggest trying the easier fixes first. Replace the thermostat and make sure the coolant level is correct. I've solved problems similar to yours on two F-150s and one Explorer doing only the t-stat and topping off the coolant. If that fixes the problem, you can then change and flush the coolant, which sounds like it needs to be done anyway. :-)
Something to watch out for... if your coolant level is already full, drain some off before opening the t-stat housing. Otherwise, coolant can run into the spark plug well for cyl 5 (driver's side front) and make a mess!