Car shaking after new tires

I had two new front tires installed on vehicle at WalMart.
After installation, front end of car began shaking and pressure light came on when reaching speeds of 50+ MPH. This had never happened before the new tires were installed. I checked the pressure gauge on all 4 tires; all 4 were fine.
I took the car back to WalMart where they double checked their work, checked the balance on all 4 tires, and checked their thread work on the lug nuts. They said everything was fine.
Car still shakes at higher speeds. Sometimes it increases when decelerating.
Any thoughts on what the problem could be?
Thank you for your time.