re: grand jeep cherokee vibrating
put it in park and rev the engine. If it still vibrates throughout the rpm range in park, then its likely something with the motor. Motor vibes can occur when there are bad motor mounts or if there is a cylinder misfire due to bad spark plug, clogged fuel injector or plug wires.
If it happens only when accelerating but not when decelerating or coasting, it can be either driveshaft u-joints or differential problems like gears or carrier bearings. 4wd models have had issues with the front differential vibrations.
If it vibrates when moving regardless of acceleration or deceleration, check tire balance.
Hope this helps somewhat