Hmm -- a few ideas come to mind.
- I'd start with making sure the appliances are level/balanced. Adjust the feet if necessary.
- I second the rubber mat idea. Your primary goal is to reduce the transfer of vibration, and that's a solid way to do it.
- Addressing the wall is the third step. In my experience, the trick with noise reduction is layering dense, rigid material to reflect noise and soft foam to dissipate the energy. So personally, my first attempt would be placing a heavy MDF board against the wall and egg shell mattress foam between that and the washer/dryer (just make sure the foam can't get wet and mold). For reference, that's exactly how I built a serviceable sound-proof box for some lab experiments at work. Or if you're up for a bigger project, doubling the drywall in the wall and filling it with insulation would probably help a lot.