My guess is your blowing a supply right at the return, otherwise there is no possible explanation for an 8 degree difference in a matter of 36" in hight in the room. With a 69 degree return, it is likely you are frosting the coil or at the very least running a low suction and now this clown has decided to add some refrigerant to get that suction pressure up leaving you with a system failure right around the corner and a lack of capacity. I'd call him back and show him the temperature difference or have his boss come out.

Something has changed, its not a unit issue. Maybe a duct is blocked or broken, until someone climbs under that house, you will never know. I suggest you get a pair of cover-alls and a flashlight and start exploring your crawl space, you might just find something else and nip that in the bud while your looking for broken or uninsulated ducts..