1984 300SD

when the car reaches about 55-60MPH the front end would shake. this only happens on straight ways. as i turn the steering wheel either left or right (to change lanes) it would go away. and as soon as it is straight again it would start shaking again.

i changed one of my upper control arm (it was loose and i thought that was the problem) but it didn't help.

what else should i look into to make it longtrip worthy?

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05