1999 Ford Explorer V8 shaking and vibration noise

I have a 1999 Ford Explorer V8 Eddie Bauer with 99,000. There is a noise and vibration when you get to about 40 mph. It doesn't happen all the time. The noise and vibration sounds just like hitting the rumble strips on the side of a highway. The car shakes for about 1 second or less. It never does it at highway speed or low speeds. Sometimes it seems to happen more if the car downshifts and then you accelerate again. Many times you can go from 0 to 60 and never get the vibration or noise.
Any ideas. Thank you