Title says it all:

When I come off the highway, and slow down at the off ramp, it makes a big CLUNK somewhere toward the rear, but maybe under rear seat. It does no further clunking, and does it at no other time.

My guess is it might be a U-joint, but I don't even know if this car has U-joints. It sounds like something has been wound up and is releasing now that the constant "pressure" (torque?) is no longer being applied. My other guess is maybe something with the exhaust.

I already checked for loose items in the back.