wheen at about 50 mph if i step on the gas kind of hard to accelerate, there is a 1-2 second crunching noise...what is it?

I Have A Similiar Problem With My 1997 Explorer 4wd. When I'm On The Freeway The Tranny Seems To Want To Downshift From Overdrive For Just A Split Second, And The Result Is A Sporadic, Loud Clunking Noise And Jerking Action. I Have Had The Tranny Rebuilt And I Have Had It Into The Ford Dealer. 1500 Dollars Later I Still Can't Drive On The Freeway. Anybody Got Any Ideas?