I just recently got a 98 blazer with 149k on it rides good. Had a few things done to it as far as maintinence. No one really took care of it before me, so you can only imagine, everythings fine now except for a loud vibration in the car betwen 40-55mph its peak at 45-50mph as if I'm riding off the express way onto the safety bumps or whatever they're called, or if I were driving over a washboard. Doesn't make this sound at any other time. And stops if I take my foot off the gas. I'm hearing bad tires? Back end going out? I have no idea, however I have yet to replace any tires, and I just had work done to it...any sugestions, please help. I'm new to the car ownership world.