High speed vibe is unlikely to be related to shocks.
Wheel rotate test good idea. May have a bent wheel, which cannot be balanced. Most tire shops do not test for this, since it requires a second 'spin' on the machine.
Otherwise, something might be getting loose, like balljoints or anything in the steering rack. Loose bearings can cause this as well (not so common). The alignment folks should have caught any of this stuff, but from my experience, they often miss things.
Since your old tires were worn unevenly, that is a clue that something was wrong. Where was the wear? Did you get the alignment printout?
2 lane roads usually have a steeper crown, and lean to the right. Divided highways have a gentler crown, and lean to the left in the left lanes. This could be another clue.
1986 300E 5-Speed 240k mi.