Jeep Grand Cherokee Shakes when you start it

My jeep has not been starting up good lately. We've
had it in the family since it was brand new and
haven't had hardly any problems except within the
past year or 2. When you go to start it, the whole
car just shakes. It normally only does it when you
have the air running when you start it. We think it
may be like an idling sensor but we really don't
know. It does have high mileage