Welcome to AF Johnnyrockson!!
The simple cause of this "clunk" could be a fast idle due to a foot on the gas that is unintentional.
Another cause is the position of the drive train that sets things up to allow the greatest amount of free play that needs to be taken up before the vehicle moves.
The ones you don't want to hear about are the ones about wear in the axles and transfercase..........
Since this is a sometime thing, it's likely a combination of the first two.....
Next question is - How long since a transmission, transfercase and diff oil change???
You can checkfor wear materials in all of these by draining the oil into individual clean containers and letting them sit for a day or so. drain off the "clean oil on the top and look in the remainder on the bottom.....
I take this a bit further and dilute the small amount of oil remaining with solvent and let it sit overnite and redrain, use a magnet to check and what is left is aluminium or brass or dirt...
You'll likely find a little bit of material there but if it looks like aluminium paint or a silver mine at the bottom - better have a deeper look....
your likely ok for now.
JD