00 Cougar 2.5

RPM's rev up to 4000 then shifts, when on Highway the car drive at 70 mph's and stays there till i let off the gas. sometimes the "transaxle" check light will come on and when that flashes, i can not turn my O/D off or ON and .
When I start the car the car is automatically in OD. is that supposed to be correct?
Is there a OD sensor or anything i can try and change to see it is the cause of all my transmission problems?