I have an 02 Bonneville I've changed tires, rotors, brakes, bearings had front end lined wheels balanced and I continue to get a vibration/shaking when braking not so much at normal speeds but very bad at highway speeds...could bad engine mounts cause this??..or can someone tell me what might be causing this...I have no clueonfused

check the control arm bushings

I dont' know about engine/transmission mounts allowing a shaking.
Did you check the CV joints and the axles for play? But more I'd think of dragging calipers.

A dragging caliper heats up the rotor while running, then when you press the brake there are high spots and the caliper catches on those due to the rotor'* warping from the heat. I'm not clear if it'* resin deposits on the rotor that are normally passed when breaking in a new set of pads and rotors or if it'* the rotor warping when hot.

I'd recommend either checking both calipers for easy sliding away from the rotor surface and lubricating the slide pins very well OR replacing both with rebuilt calipers.

Some diagnoses to distinguish that it is the fronts would be to use the emergency brake carefully and in an open area to see if it'* the rear rotors heating up and grabbing. You can apply the pedal slightly and it catches. Then you can depress it more and as you do that the catch is released so the emergency pedal can be used like the regular pedal but more dangerous because the rear wheel might lock up before you catch it. But what you are looking for is a catching as the car moves at the speed where you feel the roughness.

The rear and front tires can be interchanged to see if there'* a change in the shaking due to road force imbalance or belt problems.

thanks for all the suggestions....changed both control arms (bushings were bad) problem solved mechanic wanted $400 brought parts online for $105.. 30 minutes to change each side ..now do you recommend an alignment??

alignment should be checked because thats the bottom reference for the front end spindles and such