1986 300E 143K mi. About 500 miles ago developed vibration in front
end above 50 mph increasing to 75 mph-haven't gone higher. Increases
slightly with acceleration but goes away with deceleration, goes away with
coasting. No vibration with breaking. Vibration increases with speed
but remains the same whether acceleration is in 3rd, 4th, or 5th gear,
whether 3500, 2500 or 2000 rpm. At 60, coasting out of gear, no
vibration at 5000 rpm. Tires balanced after vibration noticed, no change.
Any
ideas?

Your initial thoughts of wheel balance, brake rotors etc are good and
the usual first aid for vibrations. You've eliminated them as choices.
I believe your car has a broken motor mount which allows the motor to
lift off that mount from the torque it produces to push the car at the
speeds you mentioned. Note that the vibration ceases when there's no
power demand and increases whenever the power is applied. Suggest, as a
test, you look at the motor (from the side) while an assistant runs it
up against the brakes - say, a brief push up to 2K rpm in D against the
brakes - you'll see the motor twist a bit (that's OK) but not more so.
Hope this helps you solve it.

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I would say check your tires... possibly broken belt, bubble on side of the
tire (bubble can be on either side of the tire)... This is safety matter so
get it checked before it blow out.