98 Chevy Monte Carlo Vibration & Sputtering

I have a 98 Monte Carlo 3.1L with 107K miles.

While driving 35+ miles/hour, I get a vibration in the front end. If, for example, I'm driving 45 mph and the vibration/sputtering is happening. When I take my foot off the gas, the vibration in the front end stops. The car rides smooth. This does not happen all the time, very intermittent. Auto repair mentioned that my engine mounts are worn more than usual.

Check Engine Light has Never Been on. No Codes or pending codes.
Repaired list: Passenger Hub replaced, Tie Rods Replaced, Transmission checked, Fluid Flushed (Fluid did not look bad), CV Joints checked, Drivers Hub Checked.
Intake Gasket replaced, New Plugs & Wires (AC Delco), new Fuel Filter, injector clean/service, MAS swapped with known good one.

My next mode of attack is to look at the Coils, or the ECM.

My question is, where is the ecm located on this car? Would I be able to swap an ecm from a 98 Malibu 3.1L into this car? Testing purposes. Of course, if you have another suggestion, I am all ears.

Thanks

I have the exact same problem. Same car, year, 130,000 miles. No one has been able to tell me for sure but maybe a cam shaft syncronizer. Does this make sence?

I have run into a problem with another make/model car that would vibrate/jerk when it reached a speed high enough for the transmission to kick into over-drive. Sometimes the switch on the transmission goes bad and overdrive doesn't properly engage. I'm not sure if the Monte Carlo has the same switch but I do know that the VSS (Vehicle Speed Sensor) can cause engine/transmission miscommunication problems that may lead to your root cause. Try driving with the shifter in the non-overdrive gear. Good luck and let me know what the problem was when you get it fixed.