I replaced the tie rods, had an alignment, tires are both equal and almost new. When the motor gets to about 65mph there's a subtle vibration and remains close about the same frequency at higher speeds but with some improvement. I feel it in the seat and in the steering wheel. I know its going to be something that will cost plenty of time and money to track down, but if I were to troubleshoot this problem, what would be the least to most expensive parts to replace?

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1985 300TD Turbo Diesel Wagon
273,000 miles