Wheel shake while braking

[switchman]

I recently noticed wheel shake when i tap brakes to slow down from about 85 mph but does not have the shake from 65 mph and below. i have not tried braking hard from high speed run since those times i had to tap brakes was just to slow down a bit.
Dealer replaced pads/drum as warranty work around 36k miles and i had put in about 6k miles now but i don't see the problem as result of brake job since braking is good during slower speed braking. I also don't feel any vibration at any speed so i don't think its wheel balance problem.
Has anyone had this problem ?