Trailer Tire Speed Rating 91 Chevy G20 Vibrating Problem At Higher Speed?

91 chevy g20 vibrating problem at higher speed? - trailer tire speed rating

I am a 91 Chevy G20 in there with a 5.7L 350th Everything was working until the film and are used for towing a trailer with a full load, even after the move everything was ok. He sat down for maybe 2 weeks without moving, and I went into the shop and began to drive, everything was going well until I reached about 60 years and older. It starts to swing, many people told me that he was discovered fat tires, but I looked at my tires are very good and she begins to vibrate when I press the gas pedal, but when I left gas stops it. Someone knows what could be the problem? Need help