Thunk when downshifting under load?

[AGodlyCanuck]

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.

 

[RangerX]

It's easy enough to check the trans futile level, add some if it's low, if it's not, don't add.
Next I would get underneath and try to rotate the driveshaft by hand, checking for free play. Sloppy worn joints might cause the issue.

 

[Rainier]

My 1995 Ford Ranger, 4x4 auto, 4.0, it clunks when switching into reverse sometimes, and from reverse into drive. Definitely audible, but not super loud like a backfire or anything. I wonder if it's the same issue, just yours is further along.