Hi .
When did the problem start. If it started after some work was done It could be that. A vibration at a given speed is usually a harmonic IE a lot of little things not quite right all coming together at the same time. The spring shock combo could be not optimum + tie rod and idler arm ware + slight wheel balance prob and wheel run out all coming together at the same time and same speed.
Also you mention already replacing the springs . check the ride height left and right , cause there is a guide plate up inside the spring tower and sometimes the new springs get caught on the wrong side of of causing all sorts of clearance and driving issues.