Are you saying you still have the winter tires on the back and the summer tires on the front? Unless all the tires are the same brand and mode, it’s possible there is enough difference in the actual size of the tires to cause the problem.
From personal experience, I put two replacement tires on my wife’s 86 Dodge Colt. The car shook violently at speeds over 40mph due to the size difference between the old and new tires (both sets were 155-75R15). A set of 4 matched tires fixed the problem.