I have a 07 Liberty Sport all stock with 65K miles on it. This week I started getting a very annoying vibration commencing at 45 mph. It seems to diminish above 60 and also seems to get a little worse when you step on the gas. You can feel it in the seat of your pants and in the steering wheel. Could it be U-joints? Front drive shaft? Can you give me a list of things to check and how to check them? Thanks.