Not necessarily. Sometimes it balances out a bit at higher speeds. You
should probably have the rotors checked for runout and thickness
variation. Some rotors distort under heat (highway braking) and may
test OK in the drive but bad on the road.

It's possible that it's a slight wheel balance or other similar
problem. When you step on the brake it tries to stop the out of
balance condition (by nature of the design of disk brakes) and you can
get vibration feeding back.

More likely it is the brakes.

Bob

My first suspicion is warped rotors. I had this problem on a car I
bought but they were so bad there was vibration at any speed. The other
things I would look at are the CV Joints, ball joints, shocks
(especially the rubber bushings), axle bearing.

Jim
'87 C900s 247K