1999 BMW 525i speed dependent vibration

1999 BMW 525i mileage: 97,000. I recently purchased what appeared to be an exceptionally well maintained 525i, which at the time of purchase had identified what appeared to be a wheel balancing problem. As it turns out this did not rectify either a mild shudder, which is evident throughout the vehicle, or a speed dependent vibration, which is both, transmitted through the steering wheel and the seat of your pants. Subsequently, the entire suspension system and drive train has been checked on several occasions, and the BMW mechanics have replaced the drive shaft, which had no impact, followed by the left rear axle, which appears to have removed the general vibration, which was evident at all, speeds. What remains is a persistent vibration which comes in at 55 mph, becomes more violent as you approach 60 mph, and then tapers off, but remains at a lesser degree of severity. The BMW mechanics appear to have no solution, and are unwilling to replace either the tires or the wheels. For my money, the problem is either one or two slightly buckled rims (two rims are well scuffed), or out of round tires, or a combination of both. I have no way of confirming this by way of a conventional balancing machine, most of which do not exceed 30 mph. Basically, my question is, where do I go from here? Any assistance would be most appreciated.