shaking front end

Hi, everyone i have a 94 plymouth voyager and the front end is shaking side to side. i have took it in to a couple of shops and said it was the tire. could that possibly cause the shaking, the tires are in need of change aways cause there arent much tread left. and alos i have p195/70r14 on them now and i just check the manual and it says it needs p195/75r14 tires. if i change them can i change them to p195/75r14 and keep the p195/70r14 on the back. will the different size affect the car ride. thxs