Hi, something about that dosen't sound right. If it was a belt tensioner you would most likely see a power loss in steering ability, alternator output etc...due to a loose belt/ belt tensioner. I would think that if you had a bad/worn tensioner you would be able to feel it even at idle(or revving the engine at a standstill in neutral) since the tensioner could be slopping around on its mount. your vibration could be a worn CV axle, or a suspension part. Did they check those parts?? good luck, Jim