I have a 94 F-250 with a 351 and 4-speed auto that's just developed a vibration, actually it's more of shake. I think I can feel it as soon as you start the motor but I can really feel it when you put it in drive. It shakes the entire cab. Place it in neutral and it's not as nearly as bad. I'm thinking it's a motor mount or possibly a transmission mount. But when this first appeard it started not to be idling smoothing, sounds like it could be missing which would drop the power but I don't think would cause a vibration. So maybe it's something internal to the motor like a balencer. Any ideas or suggestions of things I could look for/test myself before I take it to the shop? Thanks.

 

I'm thinking torque converter/broken flexplate. Either the torque converter bolts have come loose or the flexplate has broken (yes they break it happens they crack between the holes).