So when I was driving my explorer home after just purchasing it i had to take it up a noticible grade I decided to take it out of OD and put it in Drive. I kept my foot in it as I always would but when it shifted down holy hell there was a crash, A loud thud. Now if i take my foot off and downshift then nothing, it would be normal. smaller hills, nothing. Could I need to add more tranny fluid or should I be looking elsewhere? This is the only SERIOUS thing I can think of that I need to address.