Vibration

I had vibration problems for about 4 months now and i went throught everything including harmonic balancer, wheels, tires, engine mounts and transmission mounts etc etc. i almost gave up finally someone told me to turn the driveshaft 180' and reinstall, so i did it and now my vibration is gone??? how does this make sense???
My car always vibrated around 70 mph, 3500 rpm. now it doesn't weird...
any input on why this makes sense???

I checked the u-joints when i had the driveshaft out and they had no play. i have a aluminum driveshaft by the way.

Thanks

No. it was on my old car and i transfered over to the new car. but it had very very low miles on it. I and ford tech inspected the u-joint and it was almost new. It's weird how rotating the driveshaft 180' gets rid of the vibration??? what is the science in this??? Anyways it worked...