2002 Ford Explorer - Neutral Safety Switch

I have a 2002 Ford Explorer, V8 - Eddie Bauer, 73K miles, purchased used not sure of maintenance. Problem - sometimes it is hard to get the car to shift into drive. I have noticed it when there is a problem and I am pressing on the brakes, the brake lights are not on. I started slowlyu pressing the brakes and I see a flicker of the brake lights during the moment of flicker, the car will shift into drive or reverse. While driving, sometimes the brake lights are showing when I am pressing the brakes. I was told to add brake fluid, change my brake lights, or it may be the neutral safety switch which is gone bad and\or the brake lights. What is it?