Check your rear subframe. The mounts that hold the sway bar up may be broken making the sway bar bounce up and down making a clunking noise. I have the same thing in my 99 SVT! I have all the parts to replace but I dont have time. Im also located in Jersey City just incase you need help!
Also a Simple weld can temporarily fix the issue and be fairly cheap. But this is temp so when you hit another big bump it might just break again! You might have to get a reinforced rear subframe and replace it as a permanent fix(or thats what i would do)! Ive had this sound for close to 4 months without it leading to any major issues yet. If this is the problem u dont have to worry about it but the sound does get annoying and im planning to change mine soon.