If it's more noticeable while making sweeping turns on the highway, braking, or transient events that load / unload the front suspension, I'd also guess front torsion arms, and / or balljoints on the front control arms.

If it's when driving straight, comes & goes at certain speeds, changes with throttle input, changes with rear height / load in the trunk, then I'd put more money on U joints on the rear drive shaft (which can also damage the CSB & Guibo if left to where the car becomes undriveable)...