It's a good idea to do in pairs, as long as doing so is not cost prohibitive. I recently replaced both left and right complete tie rod assemblies on my 300E (included inner and outer tie rod ends AND the actual rods for BOTH sides) for only $47 shipped. FastLane has some excellent prices as well, click on "Buy Parts" above and check them out.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280