My 85 300SD has the exact same issue that you are describing. I think its something in the suspension. Its something small that causes the tire to wear uneven and get progressively worse. It also has effected one rotor that pulses when braking. I replaced just about everything in the front end except the center link(the next item on the list) and still have it.

Throwing a new tire on will make it go away for a few months.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

Mine didn't change with tires, and my tires never wore unevenly

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL

From what I read on here there are two major causes of this 60mph shimmy. Guide rod bushings and Lower Control Arm bushings. Of course I have replaced both of those and I reduced it greatly. When I replaced my tire I thought I had fixed it, but I still have something else.

I will say this. If you are going to replace the LCA bushings you might as well replace everything. Also watch out for the Harbor Freight C clamp press as it is potentially dangerous. Mine had a section of the screw shatter flinging chunks of metal out, one of which cut me.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

First checkup at my favorite Benz nazi shop since 12 months ago (yeah, owner does it his way and doesn't appreciate commentary, but damn, they do good work and can tackle and guarantee anything!) and sure enough, one LCA bushing is bad. Noticed the bearings were loose up front (told em that's what I suspected!!) and they re-seated the bearings and balanced the tires again. Problem solved. Also feels more settled over bumps. Next up: LCAs (apparently can't replace just bushings in a 124.034)

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL