I have a 1996 F-150 5.0L KingCab. It has a vibration at 36-42 mph and then again around 70-75 mph. The Ford dealer replaced the spider gear shim in an effort to fix this and it did for 12K or so miles. Now it's doing it again, so I think something is make be breaking the shim. The slower speed vibration is a cyclical shudder. The high speed vibration is more or less constant. I have a friend who thinks that it may be driveshaft phasing. Suggestions?