bullitt2655

When I an am towing (car on trailer approx 4500#) I am getting wheel vibration over 60mph, evens out around 65 then gets worse. The truck runs smooth as silk when not towing. I have the stock rims and Michelin tires. Before the tires were rotated, it felt like the front, now feels like rear.

Is this most likely a simple wheel balance situation, or could something else be causing it?