Ok, here's my problem. I have a 97 F-150 with a serious vibration problem while getting on the gas between 50 - 60 mph. Above or below that it smooths out. I have had the driveshaft sent in for balancing and they said everything to include the u-joint in the 2 piece driveshaft are good and balanced. Funny thing though? The vibration changed after re-installing the driveshaft. It's still there, it just seems different in pitch and severity. It has been leaking quite a bit of tranny fluid for the last couple of months which blows back underneath the vehicle and generally making a mess. There are no problems with the operation of the tranny. It shifts smoothly and there seems to be no loss of power. I just have to keep dumping tranny fluid into it every week are so. I have had several friends try to indentify the leak source but no luck so far. It's really a mess underneath the truck with all the fluid. Could the vibration be something to do with my torque converter?

 

Just a thought, but you may be having a bearing or bearings going bad in your tranny. When some bearings go bad they will shake quite a bit at certain rpm. This would also put a stress on the seals causing the fluid leak. You may want to have someone check out your tranny. Good Luck.