could it be from needing rear tires?
it vibrates badly until 35mph then at 57mpn, loud inside but not heard outside. on 1992 Chevrolet G Series Van (G20)
Do you feel the vibration in the steering wheel if so the front tires are the likely cause, if you feel the vibration more in the rear it is likely to be the rear tires "out of balance". Visually look at the tires do they look OK look evenly worn? Go to a tire store ask their opinion and perhaps get the tires rotated and balanced as required.
I am in need of tires: rear are a must asap very uneven wear. Tire store says I need four---but dont have the money now for four. I am hoping vibration is from need of tires and not transmission. I never knew that bad tires could cause such a bad vibration. Thank you