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1994 Buick LeSabre Custom 3.8 Litre V6. Smooth when idling in Park, rough idle in gear?

1994 Buick LeSabre Custom 3.8 Litre V6 with 103277 miles.

It runs smooth when idling in Park, rough when I put it in gear when I am stopped (either drive or reverse) has hesitation when accelerating, it smooths out again when driving then it starts to drive rough at about 55 MPH.

Mechanic told me it was the plug wires. I replaced those and the plugs. This fixed the hesitation when accelerating issue, but the other issues remain (now even seem a little worse) I thought the plugs and/or wires were bad, so I rechecked the gauge on the plugs and traced to make sure I had the wires hooked up right. Everything seems in order.

I have added fuel injector cleaner, and changed the oil, oil filter, spark plugs, spark plug wires, air filter, flushed the radiator, and changed the thermostat. (For a tune up since it has admittedly been a while)

What could be my problem?

Put a floor jack with a 2x4 under the oil pan, raise it slightly and see if it smoothes out. Might have bad motor mount