I've been getting some grinding from the back rotors. Took the tires off and for sure one of them is warped while the other is heat scored. Friend took a caliper to them and they are just under 9mm of thickness.

Do I take these to a machine shop or should I get new rotors?

looks like you are already close to the wear limit. Haynes guide states 0.331 in. or 8.407mm wear limit for an 850. Replacements don't cost that much from FCP Groton. Get new pads too and then you'll be done. DIY brakes are pretty easy so post back if you want URL's for instructions.
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'97 855na, Mobil 1, K&N, various IPD's and eBay's

Yeah, a guide would be great...in particular instructions about caliper pin retraction. I've already put in an order for new rotors and my roommate will be giving me a hand in swapping them out but he wants to know if there are any idiosyncrasies to deal with?

Here's a few URL's
http://www.freewebs.com/dacvolvo/brakediscsandpads.htm

http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?t=6774

http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?p=26225
essential for removing rear disk under adjusting the parking brake shoes

Haynes guide

http://volvospeed.com/Repair/jackingpoints.php
http://volvospeed.com/850fbrake1.php
http://volvospeed.com/850rbrake1.php

http://au.geocities.com/ozbrick850/brakes.html

http://www.volvoclub.org.uk/tech/service/850/BrakeSystem.pdf

There's more out there, but this should work for now.
Good luck & have fun
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'97 855na, Mobil 1, K&N, various IPD's and eBay's