Vibration when shifting gears ( sometimes)

[youneek28]

Ok I have a 1998 explorer 2wd V8 and automatic. It has 120000 miles and runs great. Except for the vibration I feel sometimes. It only does when the gears shifts up and down. If I step on the gas and it down shifts; it does it then as well. This is intermittent. Sometimes is doesn't happen at all when going through the gears. Sometimes the vibration will rattle the teeth out of your head and other times its barely noticeable.

In short, why would it sometimes vibrate when it shifts gears??

Thanks