Yes, that's the right part. The little linkage arm has a bushing in each end that eventually wears out; the play in the worn bushing makes the clunking noise.
Theoretically, it shouldn't matter if the car is jacked up or not, as long as it is jacked up on both sides or neither side. The sway bar functions by twisting as one side of the car becomes higher or lower than the other side; the more the car tilts, the more the sway bar resists further tilt. If the car is tilted - that is, you jacked up one side but not the other - then the bar is twisted and there is pressure on the sway bar linkage arms, so you can't tell if the bushings are worn or not.
The easiest way to tell is with the car sitting on a level surface: get down near the rear tire, locate the sway bar arm and try to wiggle or move it. If it's worn, you'll be able to wiggle it and it will make the same noise that you occasionally hear from inside the car. It's very easy and inexpensive to replace, but you have to jack up the rear end of the car (both sides at once) and remove the wheels. Then, the linkages come out with one bolt on each end.
'83 240D "Steiner"
'00 New Beetle TDI
P.S. a mechanic that I *USED* to use tried to tell me that the noise I heard was actually my rear differential failing. bah.