I thought I'd throw another data point in this thread since it's all about the same subject. I knew these parts under a different name: rear track arm (even though the very similar counterpart in the front of the car is called a trailing arm).
Anyway I went with these
Porsche 986 987 Boxster Boxster s 97 10 Adjustable Rear Track Arm Arms | eBay
Not because I really needed the adjustment (though it certainly won't hurt) but because the pair of them were cheaper than a single OEM from the factory.
Rather than just replace the track arms (my left one was rattling on tar strips) I also replaced my lower control arms and my sway bar end links and bushings.
To the question above about whether you can swap them out with the car on a ramp I'm sure you could. I found it handy to have the whole mess free to swing back and forth while I got the bolts all installed.
Prior to installing the adjustable arms I pinned each of them to the arm I removed so that I could set them to the exact length and lock them down.
One issue with the adjustable arms is the fact that they're straight instead of having that dog leg in them like the originals. To answer anyone's question about why they might have a bend, it's to clear the inside forward edge of the tire. It's close enough that I'm ordering a set of wheel spacers just to make sure they never rub.
Mine is all back together so it's on to exhaust and then lift the front where I've got another set of lower control arms and complete tie rod ends (all OEM) and off to the alignment shop.
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