93 Olds 88 overdrive rumble / vibration solved

When: 50-60mph, just as the car drops into overdrive and the RPMs drop to 1500-2000.

Symptoms: Hesitation, rumbling, front end vibration, car lugs as you step on the accelerator.

Cause: In my case...a bad spark plug wire causing only 5 cylinders to fire.

Fix: Replaced all the spark plug wires. $20.

I had suspected everything from bad CV joints, to loose engine mounts, to a bad transmission.

Mine does the same thing. Gets to about 60 tho and starts vibrating.
I need to replace the spark plugs and wires too anyways.