If you have already checked the above mentioned items and they are good, you may try adjusting the rack damper bolt. I adjusted mine about a half turn last weekend and it took care of the shakes. Do a search for rack damper bolt and there is a ton of info on it.

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2005 Accord Hybrid (Wifes)
1995 Subaru Impreza L AWD (Snow car)
1984 GMC Sierra 1500 (Mine)
1983 300CD Best $ I ever spent. (Mine)
1984 190D (sold and glad I did)
1983 300D (sold and wished I hadn't)