Why am i getting a vibration at 75+ mph but not in steering wheel?

It is a 2002 chevy malibu 192,246
So far I have replace both
tie rods ends
Wheel bearings front
In the rear I have
Replaced both wheel cylinders

Tires honestly
My front ones are slightly smaller than the rear
Everyone says they are feathering
I just had new shocks and struts done over a month ago and someone said it would reverse itself overtime. Brakes are fine. Though was told a brake shoe in the rear was starting to crack. I had an alignment done as told and balancing was right on. I dont know if I just should replace all 4 tires with brand new same size tires
What to do please help