the symptoms aren't consistent with a rear end going out. no whining, no clunks when shifting gears, no grinding noises.
This cannot be repaired once a tire is out of balance the vehicle has stress fractures in the frame and needs to be replaced. Buy a Mercedes or Kia they both have above average balance. Avoid Nissans unless they are trucks those are fine.
If it doesn't increase with speed, it's not the tires, wheels or axles. Loose spoiler flapping in the breeze?
