Take the belt off and drive it a short distance and see if shudder is gone. If it is then the shudder is related to the belt, the tensioner or an idler. While the belt is off, check the pulleys for any play in any direction you wiggle them. Very very slight play is ok, like about 1/32" or slightly more, but if more then is bad even if not causing the shudder. Turn them in your hand and see if feel any roughness. You should feel none. If all that checks out then the tensioner could be the problem. The old tensioner has sat in one spot for many miles and now when tension is released to remove belt, the spring may have given up. I am assuming you had no shudder before the alternator change. How do you feel the shudder? Floorboard at your feet? Seat of the paints? Accelerating? Just at idle? Steering wheel? through the body of the vehicle?
