Vibration at 55 mph, but is fine at 50, or 60 and over

Mechanic replaced a drive train just prior to my buying the 2002 PT cruiser from him.
I test drove it before buying but didn't get up to 55 on my back roads. When I took it out on highway it started shimmying up front to making my teeth chatter and voice warble. This was at 55 mph, but when I got past that mark it straightened out a bit and ran smoother at 60. I didn't sustain that speed though and was afraid to push it to 70. I took the car back to him and watched as he balanced all 4 tires. That wasn't the problem. So he swapped out the tires from my 2003 PT thinking the rims were the culprit. No difference.
I asked about alignment. He said he had it aligned at a shop after the work he'd done on it.

I noticed too that the shimmying isn't quite so bad once the car has warmed up. But then it's intermittent.

Mechanic gave me an inspectable warranty on it. I'm wondering if it may be CV shafts from some of the things I've been reading. It only has 86 thousand miles on it, and is very clean, no rust. Must have not been driven much and garage kept. Could the shafts / axels have gotten bent while he was changing out the Drive train?

Thank you in advance for your answers.
Sincerely,
GS