vibration/weird lurching at low speeds

1991 caravan SE with 140,000 mi 3.0L V6 engine.

It was developing a vibration at speeds over 30 mph. Mechanic said all
the tires were bad. Replaced them all. This reduced the vibration but
didn't eliminate it. It's most apparent on the highway above 30mph..

As long as I drive short distances at low speed
it doesn't happen. As soon as the car warms up, and I drive out on the highway, it starts...doesn't feel safe to me.

Here's the weird part: as I slow down, car moves ahead in a kind of
lurching way - as if one wheel was square, or a different size.

After I let the car sit for some time, the next time I drive it,
the vibration/lurch is gone.

Mechanic also said it needed front brakes ( but could that cause this?)
Putting brakes on/off doesn't seem to affect it.

He hasn't been able to diagnose the lurching ( said it's an "old" car)..

Another possibility: the right outer CV boot is gashed...but I don't hear
any clicking or clunking on turns...could that cause this?

Many thanks in advance..