It could be many things, but if stays cool while cruising on the highway but overheats in stop and go traffic suspect the fan operation and radiator blockage. 

If the engine has a fan clutch, the clutch could be worn.  Safety first! If you are not exactly sure what you are doing, take it to a shop.  

With engine off but at operating temperature, the fan should be hard to turn by hand. If it turns easily then it is probably worn out. Warning, watch out for hot surfaces. 

If it has an electric fan don't touch it even if the engine is off. Just observe if the electric fan comes on when the engine is hot. 

The fan clutch on my Mustang wore out after 20 years. I replaced it with an aluminum flex fan, then the stop snd go temps went back to normal. 

Sent from my iPhone

> On Aug 29, 2015, at 6:05 PM, John Rodgers <jrodgers113@GMAIL.COM> wrote:
> 
> Please reply by P-mail
> 
> Trying to help my son. He has 2000 Ford Ranger, 4WD, 4 L V6, and it
> overheats every time the AC is turned on. Everything seems to be up to
> snuff! Any ideas as to the problem?
> 
> Thank's,
> 
> John