Car, vibrates at 37 and 42 mpg, 1994 Voyager

I've noticed something strange about my 1994 Plymouth Voyager, 3L, plain vanilla model.

I bought Pep Boys Future 2000 tires? Probably because it's one of the few with 2 ply sidewalls.

Anyways, I rotated the tires and then over the course of months, would notice a vibration at 37 mpg which would go away immediately if I accelerated just a couple of mph.

So I had all the tires rebalanced. And the problem is still there, except at 37 and 42 mph.

So I wonder, could the previous back tires have some sort of defect, like an out of round that a balance could not detect and certainly not the Pep Boys' mechanics?

Or is there something else going on, like a strange mis-alignment. But then why only at 37 and 42 mph?

The car has been inspected and the front-end is supposedly fine.

I am going to go to the dealer but I do wonder.

The shakiness is like a tire out of balance. And around the same speed leads me to believe something strange with one of the front tires.

I guess I could rotate the tires back to the original positions and see what happens.

Any thoughts?

P205/70R14 are the tires now on the car, not P195/75R14, that are originally suggested. Someone suggested better handling, except in snow, with these slightly wider tires? Maybe. I think it affected mileage somewhat, maybe different size or maybe lower rolling resistance since Pep Boys are Cooper tires which have lower resistances often.

Regret not getting cheapo Goodyear with just 1-ply sidewalls but probably better mileage. I reckon the penalty is about 10% which now is a bit, not much, but more money for gas. I wanted the safety of the rare 2-ply sidewalls for stability but I drive mostly slowly so that's not really a problem as if I had heavy loads and head to do lots of curves and so on. The car is really quite stable for a van with the