Well, I've tried searching, but the multitude of hits that come up from "acceleration vibration" are too much to read through... not sure if there's a way to search an exact phrase instead of any word w/in a phrase - but I digress.

My MCS just started a vibration issues - generally between 50-75. It's present while coasting or easy cruising, but is accentuated under acceleration (ie passing). Even slightly noticable on braking. My 1st thought is balance - but not sure how acceleration amplifies the effect. Also - how does one test which corner the vibration comes from?

I just swapped from summers to all-seasons (can't judge if the vibration was there before as the summers were pretty shot anyway when I took them off).

What else can I check for?

 

sounds like an axle if no mods were recently done

 

if it's really noticeable w/ the additional of the all-season tyres,
i'd get the wheels checked for runout and balance

 

Having had similar issues before I would check wheel balance and weights (make sure they haven't fallen off). Also make sure the tires haven't shifted on the wheel. Happened to me once when I had my tires installed, drove 200 miles then did an autocross, and came back with lots of vibration and found my tire had slipped.

Another possibility is you need an alignment, that solved vibration issues for me as well where it didn't get bad until about 75 and past 90 it diminished greatly. In that zone though, shaking was bad enough that you could see the steering wheel bouncing left and right.