It depends on your mechanic skills and tools you have, but replacing a rear drop link is quite an easy job that can be done after removing the road wheel. As well as jacking the car up to remove the wheel I would also support it on an axle stand. You don't need to get underneath the car. New drop links aren't expensive either, I wouldn't expect to pay more than £15 each in the UK. The others things I have seen that cause knocking are worn anti-roll bar bushes, and loose bolts where the top of the shock absorber in anchored.
The problem with the drop links is that it is normally under tension from the spring so it's not so easy to feel if there is play or slop in the ball joint. If you can bounce the car up and down so the tension is on and off, the movement in the joint will allow it to knock. It probably won't be enough to
see the movement but you would hear the knock, and feel it if you touch the drop link, but take care not to trap your hand.
