I've said it before but replacing parts willy-nilly can get old real quick. There are like 20 bushings and half-a-dozen ball joints in the back end alone, plus whatever is holding the 4WS motor together. Any of these plus the 4-6 million bolts in the thing could be causing the problem. Poke your head under the back of the car and have a look around, you can just about eyeball all of the bushings in the back end. Check for anything that looks squished, dried or cracked. Give anything you can grab hold of a good wiggle and see if you can find any movement. For instance, I had a real annoying clunk in the back end that sounded like it was coming from the top of the tower, turned out to be the bolts on the sway bar mount had backed out. It's pretty hard to diagnose these things while driving.
Those SuperPro bushes might fix your issue (if it is indeed that part of the sway links that are the problem), and it wouldn't be too difficult to change them over. You can get the sway links out without taking the bar or any other suspension parts out, but the bolts into the knuckle are generally pretty seized in position so you'd want some decent tools to get them out, a 1/4" socket set won't cut it. From what I could find that bushing kit is under $20, Supercheap are a SuperPro dealer too so your local store should be able to order them in for you.