Vibration from 20mph on on 1995 Ford Explorer

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Hello, I'm looking at buying a 1995 Explorer XL 4x4 in great condition except for about a medium vibration that starts at about 20mph and continues at all speeds. It starts in the front and works it's way to the back. It's pretty smooth below 20 mph and it only gets a little worse after 30mph. No vibration in the steering wheel, it's the whole SUV. Any ideas? Also, how much would it cost to fix? Thank you in advance.
(2) Answers
Could be a simple wheel balance, or a driveshaft vibration or a transmission problem. Get it inspected before buying the vehicle, just in case it is the transmission. Just search for "Ford Explorer Transmission Failure" and you might think twice.