The vibration is felt more in the back seat and goes away if your surpass 40mpg or slow below 35mpg.
Car vibrates between 35 & 40 miles per hour not in steering seems toward back on 2004 Mitsubishi Endeavor
could tire balance issue or a belt breaking in one tire
The issue most likely resides in your Ignition Coils. They cost about $106 and the front three can be easily replaced. If your car is over about 90k miles you'll need to replace the gaskets for your spark plug wells as well. These are located under your Camshaft covers. To get to the rear three you'll need to remove your intake plenume.