I have owned both 850 and S/V 70 in older and newer series. The rear shock is really a judgment call. I hauled some beer (lots of beer) in my '98 S70 when I first bought it new and over bumps the rubber stops hit so I knew I needed mine changed. I would say if more than 80K miles, change them if you're going to do the fronts anyway. A great source is www.shox.com I bought the Bilstein Touring (TC) and am well pleased with them over the OEM and they were cheaper.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.