1991 Chevy Astro 2wd vibration above 50mph

If you can, change out the tires and see if this stops the vibration above 50- MPH. I replaced the low mileage tires on my Corvette that were 11 years old and it stopped the vibration above 65 MPH. I was told the tread was starting to let lose inside the tire itself. I had the tires balanced three times and even changed tires with rims to no avail. New tires and the vibration is gone. No vibrations up to 135. It handles a ton better. Check wheel bearings also and the idler arm.