Vibration in Outlander

Past 6 months I have a vibration/shimmy in '08 Outlander when accelerating in first gear (automatic). I can feel in my foot on accelerator and when passenger seat is empty the seat belt vibrates against door panel. It is not in steering wheel or tires. No hesitation or skipping in engine, just vibration that goes away once car gets into 2-3 gear. Dealer said transmission tested fine. Naturally. Any ideas

How many miles on the Mitsu?
How’s the maintenance history?
Changed the plugs lately? How’d they look?
Has anyone checked the ECU for stored codes?

This sounds like a rough running engine to me rather than a tranny problem. But you need to start somewhere. Try checking the ECU for codes. Post them here.