The ABS sensors would be the first place I would look, when you have the problem is it on rainy days or dry days, moisture in a cracked sensor or in the connector where they plug to the vehicle harness at each wheel can cause intermittant problems, you may want to unplug them and look for corrosion, maybe use a little dielectric grease (radio shack) at the connectors, loook for frayed wires to the front wheel hubs the sensors can be removed at the front wheel hubs pretty easily for inspection.
I have a GMC Envoy that eats front wheel bearings, tell tale sign for that is the ABS starts to come on at slow speed stops.
You may want to take a look at the wheel bearings also. Usually you will hear them growling before the ABS sensor faults (my experience with BMW wheel bearings anyway)
The front tire shake could be a out of balance tire, bent rim, wheel bearing, control arm, ball joint or bushing or a combination of all of them.
the ABS sensors look at a toothed ring around the axel and count the teeth to evaluate the wheel speed, all tires should be turning at the same speed, when one is not (slipping or skidding tire)
the computer sees somthing wrong and applies ABS / traction system, one of your sensors is failing and the computer sees it as wheel slip or skid and the ABS / traction system is comming on.
Hope this helps.