The 960 rear LCA bushing is a weak link on the car. It is designed intensionally soft to dampen a lot of rearward vibrations. If you have one in-hand it's easy to see, there's not much rubber connecting the center taperd pin and the outer gland, just two small brigdes of rubber. They don't last long, I got 22k miles out of an OEM set.
The idea is to fill the bushing in with a polyurethane calking that can fill the void on either side. The trick is to not fill it too much or use harder rubber as to cause undue vibrations or compromise the ride quality. I and others found the 3M WindowWeld to work great. I just ran a single bead smoothed so it was conforming to the existing rubber busing and had no difference on ride, but will (should) last longer.
I'm sure there are harder rubber like 3M 5800 that could really make a stiffer bushing but I haven't tried that.