Hello and welcome to Just Answer.
Are you certain the noise is coming from the rear of the car?
Broken engine mounts are a very common problem on Pacifica...a broken mount will make a noise/clunk feeling when shifting from reverse to drive or drive to reverse.
Otherwise, I would suspect that this is a problem with the rear brakes. Have you had any work done on the brakes recently?
Based on the info I have so far, I recommend you have the engine mounts checked.
(My wife had a 2007 Pacifica and the front engine mount broke at only 12,000 miles.)
I also recommend to have the rear rotors resurfaced, and see if that eliminates the thumping sound. When the rear rotors are removed, have them inspect the parking brake shoes.