I would be interested to know how many miles are on the car, as a point of reference.
Also an observation that a broken or completely worn out sway bar link can make a pretty nasty clunk. I would go there first before worrying about anything else.
Of course, this is assuming you have checked and double-checked that there is no junk in the trunk (including in the spare tire well) that could be the cause.

(Voice of experience.)