1992 Toyota Previa Engine or Transmission-related shaking a

It's an all wheel drive vehicle.

I installed new plugs, cap and rotor.A new Throttle Position Sensor and even swapped a used ECM/ECU from a junkyard. Without relief.

It's not the tires out of balance.

Car used to run really well. A well-engineered vehicle.

About a year ago a mechanic installed a rebuilt distributor. It then worked fine.

Symptoms: VIOLENT SHAKING at 50-60 mph. Engine a bit sluggish otherwise. All gears work. Must accelerate at 50-60 mph to kick-in passing gear to stop the shaking!

Could it be bad plug wires? Transmission solenoid(s) #1 or #2?

Note: For years now when driving on the freeway at say, 60mph, if you take your foot off the gas to decelerate, then put your foot back on the gas you could feel a momentary 1 or 2-second "rumble" as if the transmission was re-engaging, but only under this high speed cruising scenerio.

I really like this vehicle and suspect it has many years left in it.

Please advise me as to what the problem is, what the solution is, and what it might cost to correct.

Thanks!