My vote is out of balance tire and /or bad shock .
1. Out of balance tire, slow enough the out of balance is not sufficient to produce vibration. Fast enough and things get to bouncing.
2. Bad shock, at speed everything is fine til the tire hits a large enough bump to get it to bouncing and hits its harmonic frequency and amplifies.
3. Alignment off, yes it was just done but it might not be right. My 67 dart has a shimmy at 40 to 45 everything is in acceptable shape but I'm sure the alignment is off. Above or below the speed and it drives great, no pull good road feel.
