I have a mustang 2012 v6 and at high speed around 120-140km/h it vibrates not the steering wheel but the floor had the suspension checked which was ok and the shock absorber of front was changed.
Also idk if this helps i did an allignment but the car still has an issue when i hold the steering wheel straight it kind of goes to the left side.
And also an issue when i go over humps at high speeds there is a sound of like kreek
Hope can find a good solution have done work for over 1k and annoying cant find the solution.
I think you need to find a better front end shop and have your alignment redone. If it is pulling, either you have an alignment problem, a front left suspension issue, or both. I'd also have them road force balance the tires to make sure there are no issues there.
I would check, specifically:
1. Inner and outer time rods
2. Control arms
3. Struts
4. Front axle
5. Tires - Make sure you don't have uneven wear
If they look ok, time to have the alignment done by a good shop.
You might want to check your drive shaft to make sure the u joints are good and it is properly balanced.
Fixed it got tyres balanced