Ideally you would core aerate and spread compost over the entire site. Since your site is so large, I would opt to concentrate on one spot at a time...maybe the front lawn that everyone sees. Get this area right and work on the rest at another time. The added benefit to this is that you will get to see visible results compared to the rest of the lawn that has remained untouched.
Compost is pretty easy to spread by hand...but doing two acres would likely require a machine as greendoctor said..around here you can rent them.
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