It sounds like the classic wheel balance issue. Since you say road-forcing
the tires changed the problem, I'd primarily look at the tires and wheels.
And don't be fooled about the road forcing. Putting them on the machine
and checking the road force doesn't really do anything. The only way to
actually minimize (and this is very time consuming and likely to be
expensive) is to:
1. Drive the vehicle several miles/kilometers to be sure the tires are
2. Use the matchmaker feature of the machine. This requires checking the
road force of all tires, dismounting them, and checking the runout of all
rims. The machine then instructs which tires to put on which rims, how to
match the tire position to the rim to minimize vibration, and where to
install the wheels on the vehicle to minimize vibration.
Even this won't solve the vibration issue if the tire or wheel is too far
out of balance/out of round. Start by checking the tires and wheels,
especially the tires if the problem changed when the tires were changed.