Well I'd say start with the tires, check for out of roundness, knots, balance. Then work your way in, bearings, discs, shocks, springs, hangers, anything that could be broken or loose. Pinion bearing nut loose, bent driveshaft, work forward. Put on jacks, have someone spin it to 65-75 mph give it a visual for movement. My buddy's car had a bad vibe, he thought it was an out of balance engine that had recently been built, so I got it off the ground to look, one of the c-clips had come out of the u-joint and driveshaft was flopping around. It felt like a square tire. LOL