It's almost time for my bi-annual 1000 mile road trip but there is a problem that must go before the trip. It's strange. The car rides pretty nice around town. Most of the roads around here are 50 MPH speed limit and I do 60 MPH a lot of the time (with the flow of traffic, of course). The car is smooth as silk. However, when I get on the freeway doing about 70-75 MPH and there is a pattern or "rhythm" to the bumps in the pavement, the car really handles it poorly. The car shakes up and down much the way an older full size pick-up does but with a much more unsettling feel. What is my most likely culprit here? My tires are mismatched brands but all the right/same size. The shocks are old but handle most pavements pretty well (maybe a mount problem?). I have considered the fact that I may need to check out my wheel bearings too. Please help with a little direction here. Thanks.