I have done it and paid to get it done. The companies in Albuquerque will quote you a price...but when the arrive and start work, they tell you the quote was for only one spring. It actually costs about $500 to get them replaced here.. It took them about an hour...it took me about 3 hrs. The springs that I installed cost about $100 a pair. If you decide to do it yourself, check the other parts (cables for fraying and bushings on the ends for wear) while you have it apart. They often last about the same time as the springs. The last set I did were powder coated so they will be rust resistant...but that is really more for looks than function. This is a pretty good DIY article for this task:
http://www.truetex.com/garage.htm
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74 Targa 3.0, 89 Carrera, 04 Cayenne Turbo
http://www.pelicanparts.com/gallery/fintstone/
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